[Coverage] Suppress covmap and profdata for system headers. (#97952)

With `system-headers-coverage=false`, functions defined in system
headers were not instrumented but corresponding covmaps were emitted. It
caused wasting covmap and profraw.

This change improves:

- Reduce object size (due to reduced covmap)
- Reduce size of profraw (uninstrumented system headers occupied
counters)
- Smarter view of coverage report. Stubs of uninstrumented system
headers will be no longer seen.

1 files changed, 3 insertions, 0 deletions

diff --git a/clang/lib/CodeGen/CodeGenModule.cpp b/clang/lib/CodeGen/CodeGenModule.cpp
index 44bc7fb..5c810cd 100644
--- a/clang/lib/CodeGen/CodeGenModule.cpp
+++ b/clang/lib/CodeGen/CodeGenModule.cpp
@@ -7165,6 +7165,9 @@ void CodeGenModule::AddDeferredUnusedCoverageMapping(Decl *D) {
     SourceManager &SM = getContext().getSourceManager();
     if (LimitedCoverage && SM.getMainFileID() != SM.getFileID(D->getBeginLoc()))
       break;
+    if (!llvm::coverage::SystemHeadersCoverage &&
+        SM.isInSystemHeader(D->getBeginLoc()))
+      break;
     DeferredEmptyCoverageMappingDecls.try_emplace(D, true);
     break;
   }