[Lex] inferModuleFromLocation should do no work if there are no modules

getModuleContainingLocation ends up on the hot-path for typical C code
which can lead to calls to getFileIDSlow.

To speed this up, short circuit inferModuleFromLocation when there
aren't any modules, implicit or otherwise.

This shaves 4-5% build time when building the linux kernel.

llvm-svn: 269687

clang/lib/Lex/ModuleMap.cpp

@@ -917,6 +917,9 @@ Module *ModuleMap::inferModuleFromLocation(FullSourceLoc Loc) {
   if (Loc.isInvalid())
     return nullptr;
 
+  if (UmbrellaDirs.empty() && Headers.empty())
+    return nullptr;
+
   // Use the expansion location to determine which module we're in.
   FullSourceLoc ExpansionLoc = Loc.getExpansionLoc();
   if (!ExpansionLoc.isFileID())