[6.4 🍒][Literal Expressions] Improvements to semantics to address feedback

[artemcm]

Cherry-pick of #88952

• Explanation: This change addresses feedback on the initial Literal Expressions pitch, which has also been reflected in the updated proposal.

  • Support Int128 and UInt128.
  • Add support for wrapping arithmetic and masking shift operators. &+,&-,&*,&<<,&>>.
  • Use arithmetic shift for signed >>.
    The expression folder previously applied APInt::lshr for every >> regardless of signedness. For a negative signed operand this yields a large positive value and diverges from Swift runtime behavior, which performs an arithmetic right shift that preserves the sign bit.
  • Reject references to publicly visible let bindings.
    A let binding declared public, package, or open participates in its module's ABI surface as a symbol — clients link against the declaration, not against its value, so the author is free to change the value in a future release. Folding such a reference in a literal expression would risk "baking" a module-private value into client code and remove the author's room to evolve the binding. Reject these references with a new diagnostic that reports the offending access level. internal, fileprivate, and private bindings continue to fold, as do Clang-imported constants.

• Scope: Uses of experimental feature LiteralExpressions are expanded in some ways, and restricted in the ability to reference public let declarations. Code which does not opt-in to be using this experimental feature is not affected.

• Risk: Low, does not affect code not guarded by the experimental feature

• Original PR: [Literal Expressions] Improvements to semantics to address feedback #88952

• Testing: Tests added to the test suite to validate changes to behavior and added semantics.

• Reviewers: TBD

[artemcm]

@swift-ci test