CodeGen: Introduce CodeGenPGO::setCurrentRegionUnreachable

There are a number of places where we do PGO.setCurrentRegionCount(0)
directly after an unconditional branch. Give this operation a name so
that it's clearer why we're doing this.

llvm-svn: 199138
diff --git a/clang/lib/CodeGen/CGCall.cpp b/clang/lib/CodeGen/CGCall.cpp
index 4bdd032..d312177 100644
--- a/clang/lib/CodeGen/CGCall.cpp
+++ b/clang/lib/CodeGen/CGCall.cpp
@@ -2184,7 +2184,7 @@
     call->setCallingConv(getRuntimeCC());
     Builder.CreateUnreachable();
   }
-  PGO.setCurrentRegionCount(0);
+  PGO.setCurrentRegionUnreachable();
 }
 
 /// Emits a call or invoke instruction to the given nullary runtime