[SYCL] Add C++ linkage specification in SYCL assert headers (#15570)

Some SYCL applications are seeing compilation failures related to the
SYCL assert headers. Specifically, they are wrapping the SYCL assert
headers in C linkage specifications in their applications. These headers
in turn include files that contain templates which emit errors under C
linkage specifications so this PR explicitly adds C++ linkage
specifications in the header contents to override any extra
specification that the clients may add. Here is an example of faulty
client code:

```
#ifdef __cplusplus
extern "C" {
#endif

#include <assert.h>

#ifdef __cplusplus
}
#endif

int main() {}

```
Error fragments:

```
/nfs/site/home/lbushi/sycl_workspace/llvm/build/bin/../include/CL/__spirv/spirv_types.hpp:140:1: error: templates must have C++ linkage
  140 | template <typename T, std::size_t R, std::size_t C, MatrixLayout L,
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  141 |           Scope::Flag S = Scope::Flag::Subgroup,
      |           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  142 |           MatrixUse U = MatrixUse::MatrixA>
      |           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../test.cpp:2:1: note: extern "C" language linkage specification begins here
    2 | extern "C" {
      | ^
In file included from ../test.cpp:5:
In file included from /nfs/site/home/lbushi/sycl_workspace/llvm/build/bin/../include/sycl/stl_wrappers/assert.h:21:
In file included from /nfs/site/home/lbushi/sycl_workspace/llvm/build/bin/../include/CL/__spirv/spirv_vars.hpp:13:
/nfs/site/home/lbushi/sycl_workspace/llvm/build/bin/../include/CL/__spirv/spirv_types.hpp:156:1: error: templates must have C++ linkage
  156 | template <typename dataT>
      | ^~~~~~~~~~~~~~~~~~~~~~~~~
../test.cpp:2:1: note: extern "C" language linkage specification begins here
    2 | extern "C" {
      | ^
In file included from ../test.cpp:5:
In file included from /nfs/site/home/lbushi/sycl_workspace/llvm/build/bin/../include/sycl/stl_wrappers/assert.h:21:
In file included from /nfs/site/home/lbushi/sycl_workspace/llvm/build/bin/../include/CL/__spirv/spirv_vars.hpp:13:
/nfs/site/home/lbushi/sycl_workspace/llvm/build/bin/../include/CL/__spirv/spirv_types.hpp:158:1: error: templates must have C++ linkage
  158 | template <typename dataT>
      | ^~~~~~~~~~~~~~~~~~~~~~~~~
../test.cpp:2:1: note: extern "C" language linkage specification begins here
    2 | extern "C" {
      | ^
In file included from ../test.cpp:5:
In file included from /nfs/site/home/lbushi/sycl_workspace/llvm/build/bin/../include/sycl/stl_wrappers/assert.h:21:
In file included from /nfs/site/home/lbushi/sycl_workspace/llvm/build/bin/../include/CL/__spirv/spirv_vars.hpp:13:
/nfs/site/home/lbushi/sycl_workspace/llvm/build/bin/../include/CL/__spirv/spirv_types.hpp:162:1: error: templates must have C++ linkage
  162 | template <typename dataT, int dims>
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../test.cpp:2:1: note: extern "C" language linkage specification begins here
    2 | extern "C" {
      | ^
In file included from ../test.cpp:5:
In file included from /nfs/site/home/lbushi/sycl_workspace/llvm/build/bin/../include/sycl/stl_wrappers/assert.h:21:
In file included from /nfs/site/home/lbushi/sycl_workspace/llvm/build/bin/../include/CL/__spirv/spirv_vars.hpp:13:
/nfs/site/home/lbushi/sycl_workspace/llvm/build/bin/../include/CL/__spirv/spirv_types.hpp:177:1: error: templates must have C++ linkage
  177 | template <int Bits> using ap_int = _BitInt(Bits);
      | ^~~~~~~~~~~~~~~~~~~
../test.cpp:2:1: note: extern "C" language linkage specification begins here
    2 | extern "C" {
      | ^
```

sycl/include/sycl/stl_wrappers/assert.h

@@ -7,7 +7,7 @@
 //===----------------------------------------------------------------------===//
 
 // Must not be guarded. C++ standard says the macro assert is redefined
-// according to the current state of NDEBUG each time that <cassert> is
+// according to the current state of NDEBUG each time that <assert.h> is
 // included.
 
 #if defined(__has_include_next)
@@ -16,6 +16,7 @@
 #include <../ucrt/assert.h>
 #endif
 
+extern "C++" {
 #ifdef __SYCL_DEVICE_ONLY__
 #include <CL/__spirv/spirv_vars.hpp>
 
@@ -42,3 +43,4 @@ __devicelib_assert_fail(const char *, const char *, int32_t, const char *,
 #endif
 #endif
 #endif
+}

sycl/include/sycl/stl_wrappers/cassert

@@ -16,6 +16,7 @@
 #include <../include/cassert>
 #endif
 
+extern "C++" {
 #ifdef __SYCL_DEVICE_ONLY__
 #include <CL/__spirv/spirv_vars.hpp>
 
@@ -42,3 +43,4 @@ __devicelib_assert_fail(const char *, const char *, int32_t, const char *,
 #endif
 #endif
 #endif
+}

sycl/test/basic_tests/assert_header_with_c_linkage.cpp

@@ -0,0 +1,19 @@
+// RUN: %clangxx -fsycl -DASSERT -fsyntax-only %s
+// RUN: %clangxx -fsycl -DCASSERT -fsyntax-only %s
+
+// Verify that compilation works when assert.h/cassert is wrapped by a C linkage
+// specification.
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#if defined(ASSERT)
+#include <assert.h>
+#elif defined(CASSERT)
+#include <cassert>
+#endif
+
+#ifdef __cplusplus
+}
+#endif