[OpenCL] Allow addr space spelling without __ prefix in C++.

For backwards compatibility we allow alternative spelling of address
spaces - 'private', 'local', 'global', 'constant', 'generic'.

In order to accept 'private' correctly, parsing has been changed to
understand different use cases - access specifier vs address space.

Fixes PR40707 and PR41011!

Differential Revision: https://reviews.llvm.org/D59603

llvm-svn: 356888
diff --git a/clang/lib/Parse/ParseTentative.cpp b/clang/lib/Parse/ParseTentative.cpp
index 11f1aeb..81079cd 100644
--- a/clang/lib/Parse/ParseTentative.cpp
+++ b/clang/lib/Parse/ParseTentative.cpp
@@ -1411,6 +1411,7 @@
   case tok::kw_const:
   case tok::kw_volatile:
     // OpenCL address space qualifiers
+  case tok::kw_private:
   case tok::kw___private:
   case tok::kw___local:
   case tok::kw___global: