[Coverage] Map regions from system headers (#76950)

In 2155195, the
"system-headers-coverage" option has been added but not used in all
necessary places.

This is the recommit since it has been reverted in
faef68b

Potential reviewers: @gulfemsavrun @petrhosek

Co-authored-by: Manuel Kalettka <manuel.kalettka@kernkonzept.com>

clang/lib/CodeGen/CodeGenPGO.cpp

@@ -28,6 +28,8 @@ static llvm::cl::opt<bool>
                          llvm::cl::desc("Enable value profiling"),
                          llvm::cl::Hidden, llvm::cl::init(false));
 
+extern llvm::cl::opt<bool> SystemHeadersCoverage;
+
 using namespace clang;
 using namespace CodeGen;
 
@@ -1022,7 +1024,7 @@ bool CodeGenPGO::skipRegionMappingForDecl(const Decl *D) {
   // Don't map the functions in system headers.
   const auto &SM = CGM.getContext().getSourceManager();
   auto Loc = D->getBody()->getBeginLoc();
-  return SM.isInSystemHeader(Loc);
+  return !SystemHeadersCoverage && SM.isInSystemHeader(Loc);
 }
 
 void CodeGenPGO::emitCounterRegionMapping(const Decl *D) {

clang/lib/CodeGen/CoverageMappingGen.cpp

@@ -37,7 +37,7 @@ static llvm::cl::opt<bool> EmptyLineCommentCoverage(
                    "disable it on test)"),
     llvm::cl::init(true), llvm::cl::Hidden);
 
-static llvm::cl::opt<bool> SystemHeadersCoverage(
+llvm::cl::opt<bool> SystemHeadersCoverage(
     "system-headers-coverage",
     llvm::cl::desc("Enable collecting coverage from system headers"),
     llvm::cl::init(false), llvm::cl::Hidden);