[Coverage] Strip <unknown> from PGO names if no filenames are available

Patch suggested by David Li!

llvm-svn: 264586
diff --git a/llvm/lib/ProfileData/InstrProf.cpp b/llvm/lib/ProfileData/InstrProf.cpp
index 045ba1a..2446521 100644
--- a/llvm/lib/ProfileData/InstrProf.cpp
+++ b/llvm/lib/ProfileData/InstrProf.cpp
@@ -90,7 +90,7 @@
 
 StringRef getFuncNameWithoutPrefix(StringRef PGOFuncName, StringRef FileName) {
   if (FileName.empty())
-    FileName = "<unknown>";
+    return PGOFuncName;
   // Drop the file name including ':'. See also getPGOFuncName.
   if (PGOFuncName.startswith(FileName))
     PGOFuncName = PGOFuncName.drop_front(FileName.size() + 1);