runtime: preserve line directive caller filenames

[cpunion]

Summary

• preserve adjusted call-site filenames, not only lines, in LLGo caller-frame metadata
• normalize package-loader-expanded relative //line filenames back to Go runtime spelling (foo.go, c:/foo/bar.go) and map empty directive filenames to ??
• add stable test/go coverage for runtime.Caller / FuncForPC.FileLine under absolute, Windows-style, relative, and empty filename line directives
• remove only the verified darwin/arm64 xfails for fixedbugs/issue18149.go and fixedbugs/issue22662.go

Stack note

This branch includes the caller-frame metadata commit from PR #1884 because these fixedbugs exercise runtime.Caller; without that dependency, the current mainline placeholder returns ???:1. The second commit is the scoped line-directive/source-position fix.

Testing

• Reproduced with xfail disabled before the scoped fix:
  • go test ./test/goroot -count=1 -run TestGoRootRunCases -args -goroot "$(go env GOROOT)" -dirs fixedbugs -case '^fixedbugs/(issue18149|issue22662)\\.go$' -xfail /tmp/llgo-no-xfail.yaml -run-timeout 30s
  • after the caller-frame baseline, LLGo reported the physical/package-relative files instead of the directive files
• go test ./test/go -run 'TestRuntimeCaller' -count=1
• go build -tags=dev -o /tmp/llgo-linedir-test ./cmd/llgo
• LLGO_ROOT=$PWD /tmp/llgo-linedir-test test ./test/go/runtime_caller_test.go
• go test ./cl -run 'TestEmitDo|TestExplicitDefer|Test.*Rewrite|TestOffsetOfHelpersAndSourceLineCache' -count=1
• go test ./ssa -run 'TestFuncCall|TestSetBlockEx|TestClosureFuncPtrValue' -count=1
• (cd runtime && go test ./internal/runtime -count=1)
• go test ./test/goroot -count=1 -run TestGoRootRunCases -args -goroot "$(go env GOROOT)" -llgo /tmp/llgo-linedir-test -dirs fixedbugs -case '^fixedbugs/(issue18149|issue22662)\\.go$' -xfail /tmp/llgo-no-xfail.yaml -run-timeout 30s
• go test ./test/goroot -count=1 -run TestGoRootRunCases -args -goroot "$(go env GOROOT)" -llgo /tmp/llgo-linedir-test -dirs fixedbugs -case '^fixedbugs/(issue18149|issue22662)\\.go$' -run-timeout 30s

GOROOT CI is slow/disabled in this repo, so this PR relies on the targeted GOROOT runs above plus normal PR CI from the fork branch. I did not push any branch to xgo-dev/llgo; the head branch is on cpunion/llgo.

[cpunion]

CI follow-up for 82d126f:

• Fixed the current regular PR CI failures by limiting caller-frame instrumentation to packages that actually use runtime caller metadata APIs (runtime.Caller, Callers, CallersFrames, FuncForPC) and disabling that instrumentation for std/runtime packages plus -target/wasm builds.
• Removed a new strings import from the LLGo runtime alt package to avoid pulling errors/internal/reflectlite into minimal runtime-only demos.

Local verification:

• go test ./ssa ./cl ./test/go ./test/goroot
• LLGO_ROOT=$PWD /tmp/llgo-pr1903 test -timeout=20m -run 'TestRuntimeCaller|TestRuntimeCallers' ./test/go
• go test ./test/goroot -run TestGoRootRunCases -goroot $(go env GOROOT) -dirs . -case '^(inline_caller|inline_callers)\.go$|^fixedbugs/(issue18149|issue22662)\.go$' -directive-mode=ci -progress=30s -timeout=30m
• _demo/embed/targetsbuild: bash build.sh empty /tmp/llgo-targets-smoke.txt with xiao-esp32c3
• _demo/go/runtime: LLGO_ROOT=$PWD /tmp/llgo-pr1903 run .
• go run ./cmd/llgo test -timeout=20m ./test/std/crypto/rsa
• go run ./cmd/llgo test -timeout=20m ./test/std/crypto/x509
• git diff --check

This PR still stacks on the caller-frame/debugline work from #1884; this commit makes the stacked PR suitable for regular PR CI without pushing anything to xgo-dev.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 54.14013% with 72 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
cl/instr.go	47.70%	51 Missing and 6 partials ⚠️
cl/compile.go	71.73%	11 Missing and 2 partials ⚠️
ssa/package.go	0.00%	2 Missing ⚠️

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