cmd/compile: lower Go runtime type data and interfaces to LLVM

[zhouguangyuan0718]

Summary

• lower compiler-collected Go data into typed LLVM globals, preserving GoObj symbol identity and expressing linker relations with native LLVM semantics where possible
• emit runtime type descriptors for all supported kinds, uncommon method data, method wrappers, and static itabs
• lower interface calls, conversions, assertions, equality, and type switches through the Go ABI
• document the LLVM IR-to-GoObj data pipeline and expand LLVM regression coverage to 40/95 codegen files and 19 runtime files

The codegen contract is IR generation plus opt -passes=verify and semantic FileCheck checks. It intentionally does not require O2 or llc.

Companion PRs

• llvm: preserve GoObj data semantics from LLVM IR llvm-project#4: GoObj data writer support
• go-llvm: add global metadata attachment support go-llvm#4: global metadata attachment binding

Testing

• ./bin/go test -tags="llvm23 dynamicllvm" cmd/internal/testdir -run "Test/LLVM" -count=1
• ./bin/go test -tags="llvm23 dynamicllvm" cmd/internal/testdir -run "Test/LLVM/codegen" -count=5