[Improve-optional-field-access-desugar] Desugar field access expression to if statement expression

[rdulmina]

Purpose

$title

Fixes #43769

Approach

Desugared field access expression to the following structure

// Before :
type A {
  B b;
}
type B {
 int c;
}
int? res = a?.b?.c

// After desugar :
any|error temp_result;
temp_result = a;
if temp_result !is error? {
    temp_result = (<A> temp_result).b;
    if temp_result !is error? {
        temp_result = (<B> temp_result).c;
    }
}
int? res = <int> temp_result;
 

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