Fix XML member access type and allow XML langlib calls with unions

[MaryamZi]

Purpose

$title.

Fixes #35019
Fixes #28697

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

All modified and coverable lines are covered by tests ✅

Project coverage is 77.50%. Comparing base (b45e1e1) to head (2f37ab3).
Report is 25 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #43421      +/-   ##
============================================
- Coverage     77.51%   77.50%   -0.01%     
- Complexity    58585    58591       +6     
============================================
  Files          3438     3438              
  Lines        219219   219230      +11     
  Branches      28921    28917       -4     
============================================
- Hits         169921   169909      -12     
- Misses        39884    39908      +24     
+ Partials       9414     9413       -1     

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