Allowlist for SanitizerCoverage

clang provide option fsanitize-coverage-allowlist to pass allow list to SanitizerCoverage, it passes option in code like below

static void addSanitizers(const Triple &TargetTriple,
                          const CodeGenOptions &CodeGenOpts,
                          const LangOptions &LangOpts, PassBuilder &PB) {
    MPM.addPass(SanitizerCoveragePass(
        SancovOpts, CodeGenOpts.SanitizeCoverageAllowlistFiles,
        CodeGenOpts.SanitizeCoverageIgnorelistFiles));
    ...
}

However, rust seems only can use -Cllvm-args to pass option registered by cl::opt etc., thus has no chance to pass allowlist

Start by tracing how rustc configures SanitizerCoverage and forwards coverage options to LLVM; the issue shows that allowlists currently require -Cllvm-args. Compare this path with Clang's SanitizerCoverageAllowlistFiles handling. Done means Rust can pass a SanitizerCoverage allowlist without requiring raw LLVM arguments.