[CIR] Add support for SourceLocExpr (__builtin_LINE, etc.)

[lanza]

Stack from ghstack (oldest at bottom):

• [CIR] Add support for CK_AtomicToNonAtomic casts #1989
• -> [CIR] Add support for SourceLocExpr (__builtin_LINE, etc.) #1988
• [CIR] Gut out CI configuration #1990

This patch implements SourceLocExpr code generation, which enables
support for the following builtin functions:

• __builtin_LINE
• __builtin_FILE
• __builtin_FUNCTION
• __builtin_COLUMN

These builtins are evaluated at compile-time and emit constant values
or global string literals. The implementation follows the traditional
CodeGen approach of using ConstantEmitter to emit abstract constants
from evaluated APValues.

A key challenge was handling synthetic StringLiterals created during
constant evaluation (for __builtin_FILE and __builtin_FUNCTION).
These synthetic literals don't have valid source locations, which
caused assertion failures when creating global string constants.
The fix adds a check in getGlobalForStringLiteral to use
UnknownLoc when the StringLiteral has an invalid source range.

This feature unblocks:

• std::source_location (C++20)
• Logging and debugging macros that use source location builtins
• Assertion/diagnostic frameworks

Test coverage includes:

• Basic __builtin_LINE, __builtin_FILE, __builtin_FUNCTION, __builtin_COLUMN
• Usage in global variable initializers
• Default function arguments
• Lambda expressions
• Combined usage of multiple source location builtins

[bcardosolopes]

LGTM with one minor nit!

[bcardosolopes]

Please add LLVM and OGCG checks too!

[bcardosolopes]

The windows failure seems exposed after rebase, we should probably REQUIRES linux / darwin for it right now