cmd/compile: emit LLVM source locations for Go PCLN

[zhouguangyuan0718]

Summary

• emit a line-tables-only Go compile unit with DIFile, DISubprogram, DILocation, and complete frontend inlined-at chains
• keep an exact subprogram-to-Go-symbol map for pcinline callees without adding source types or variables
• preserve abstract content-addressable inline callees as weak unreachable Go functions so GoObj still receives FuncInfo carriers
• preserve imported linkname inline callees as GoObj non-package references instead of reclassifying their export indices as ordinary package symbols
• set and restore debug locations around SSA value and block lowering so generated instructions do not inherit stale positions
• preserve the source-call DILocation when expanding explicit LLVM nil checks into compare, branch, and panicmem control flow
• use a Go-specific late MachineFunction plugin pass to canonicalize shared inline call sites and insert real post-layout anchors before branch relaxation
• verify object-level inline tables and an O2 panic traceback containing capture, inner, middle, outer, and main in order
• promote seven formerly PCLN-blocked testdir cases from graylist to required whitelist: devirtualization_nil_panics.go, fixedbugs/issue73916.go, issue73916b.go, issue73917.go, issue73920.go, inline_caller.go, and inline_callers.go

Scope

This PR only feeds pcfile, pcline, and pcinline generation. It intentionally does not generate Go target-object DWARF carriers, type DIEs, variables, dbg.declare records, or variable locations. Target-object DWARF is a separate follow-up.

Generic LLVM DWARF emission is not changed. GoObj-specific collection and inline anchoring stay in the Go handlers and plugin.

LLVM payload

• release: goallc-llvm23.1.0-20260811T022435Z
• revision: 90e5e5c7c626e3072fc77ce69cb42d8c7bb1b4a4
• contains the merged LLVM PR cmd/compile: lower LLVM LE tail calls #62 support; go-llvm PR cmd/llvmplugin: add Go statepoint and GoObj stack maps #10 is also integrated

Validation

• rebased onto go1.27.master at fa951ec and rebuilt the Go toolchain against the pinned payload with ccache
• post-rebase range-diff preserved all seven original topic commits exactly
• GoALLC plugin CTest: 60/60 passed
• cmd/compile/internal/ssa passed, including the newly merged AMD64 map SIMD lowering tests
• focused GoObj PCLN inline traceback, imported-linkname classification, and explicit nil-check runtime/object checks passed
• all seven promoted required tests passed locally through llvmtoolexec, the pinned opt/llc payload, default, GoObj link, and execution
• encoding/gob, encoding/json, and encoding/xml passed locally as LLVM entry packages after fixing imported linkname pcinline callees; the focused object test verifies reflect.(*rtype).Elem and reflect.(*rtype).Kind are non-package references
• GitHub Actions run 31553776820 passed on linux-amd64 and linux-arm64, including the 60-package, three-run LLVM standard-library gate and the complete required/advisory LLVM policy