Revamp `cg_llvm` debuginfo handling to use the LLVM-C API

[workingjubilee]

We currently use C++ code for some of our debuginfo stuff with LLVM. We should port it to the C API, so we don't have to continue maintaining irksome C++ bindings. This is the primary header in question: https://github.com/llvm/llvm-project/blob/main/llvm/include/llvm-c/DebugInfo.h

[Zalathar]

Incomplete list of places where the LLVM-C API appears to be inadequate:

• LLVMDIBuilderCreateCompileUnit
  • We pass a DICompileUnit::DebugNameTableKind to the C++ constructor, but the C API unconditionally passes DICompileUnit::DebugNameTableKind::Default.
  • There doesn't appear to be a way to change this afterwards.
  • This has something to do with not emitting .debug_pubnames and .debug_pubtypes on DWARFv4 or lower.
• LLVMDIBuilderCreateFile
  • Doesn't allow passing an optional checksum or optional source text.
  • Later versions of LLVM have LLVMDIBuilderCreateFileWithChecksum which takes optional source text and a required checksum, but that's a problem because in some cases we don't want a checksum.
• LLVMDIBuilderCreateFunction
  • Only supports a subset of DISubprogram::DISPFlags.
• LLVMDIBuilderCreateEnumerator
  • Does not allow specifying bit-width of the value; default width appears to be 64, which cannot hold 128-bit values.
  • Unclear whether this is important or not.
  • There is now an LLVMDIBuilderCreateEnumeratorOfArbitraryPrecision, but only as of LLVM 21.
• LLVMDIBuilderCreateEnumerationType
  • Does not allow specifying IsScoped; default value is false but we always pass true.

LLVM-C functions that require LLVM 19, so we can't use them yet:

• LLVMDIBuilderInsertDeclareRecordAtEnd
  • cg_llvm: Replace some DIBuilder wrappers with LLVM-C API bindings (part 5) #146763

Wrapper functions that call a C++ method with no equivalent in the C API:

• LLVMRustDIBuilderCreateMethod
• LLVMRustDIBuilderCreateVariantMemberType
• LLVMRustDIBuilderCreateVariantPart
• LLVMRustDIBuilderCreateTemplateTypeParameter
• LLVMRustDICompositeTypeReplaceArrays
  • There is now an LLVMReplaceArrays, but only as of LLVM 21.
  • Unfortunately, LLVMReplaceArrays only exposes the two-argument form of replaceArrays, but we use the three-argument form.
• LLVMRustDILocationCloneWithBaseDiscriminator

Wrapper functions that do non-trivial things and don't map cleanly to a single C or C++ API function:

• LLVMRustDIBuilderCreateStaticVariable
• LLVMRustDIBuilderCreateVariable

Wrapper functions for getting various DWARF constants:

• LLVMRustDIBuilderCreateOpDeref
• LLVMRustDIBuilderCreateOpPlusUconst
• LLVMRustDIBuilderCreateOpLLVMFragment