Eliminate BUnionType's getMemberTypes() usages

[lochana-chathura]

Purpose

$subject

Fixes #43344

Approach

n/a

Samples

n/a

Remarks

n/a

Check List

• Read the Contributing Guide
• Updated Change Log
• Checked Tooling Support (#)
• Added necessary tests
  • Unit Tests
  • Spec Conformance Tests
  • Integration Tests
  • Ballerina By Example Tests
• Increased Test Coverage
• Added necessary documentation
  • API documentation
  • Module documentation in Module.md files
  • Ballerina By Examples

[codecov]

Codecov Report

Attention: Patch coverage is 97.18310% with 2 lines in your changes missing coverage. Please review.

Project coverage is 77.40%. Comparing base (0e984bb) to head (f8d8a47).
Report is 1 commits behind head on nutcracker.

Files with missing lines	Patch %	Lines
...piler/semantics/analyzer/CompilerPluginRunner.java	0.00%	1 Missing ⚠️
...compiler/semantics/analyzer/TypeParamAnalyzer.java	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@               Coverage Diff                @@
##             nutcracker   #43341      +/-   ##
================================================
- Coverage         77.41%   77.40%   -0.01%     
+ Complexity        58285    58227      -58     
================================================
  Files              3439     3439              
  Lines            219307   219168     -139     
  Branches          28808    28743      -65     
================================================
- Hits             169773   169646     -127     
  Misses            40044    40044              
+ Partials           9490     9478      -12     

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