Eliminate BUnionType's `getMemberTypes()` usages

[lochana-chathura]

Purpose

$subject

Fixes #43344 Fixes #43437

Approach

n/a

Samples

n/a

Remarks

n/a

Check List

• [x] Read the Contributing Guide
• [ ] Updated Change Log
• [ ] Checked Tooling Support (#<Issue Number>)
• [ ] 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