Add -f[no-]declspec to control recognition of __declspec as a keyword

In versions of clang prior to r238238, __declspec was recognized as a keyword in
all modes.  It was then changed to only be enabled when Microsoft or Borland
extensions were enabled (and for CUDA, as a temporary measure).  There is a
desire to support __declspec in Playstation code, and possibly other
environments.  This commit adds a command-line switch to allow explicit
enabling/disabling of the recognition of __declspec as a keyword.  Recognition
is enabled by default in Microsoft, Borland, CUDA, and PS4 environments, and
disabled in all other environments.

Patch by Warren Ristow!

llvm-svn: 249279