Adds a warning for unrecognized argument to #pragma comment() on PS4.

PS4 target recognizes the #pragma comment() syntax as in -fms-extensions, but
only handles the case of #pragma comment(lib). This patch adds a warning if any
other arguments are encountered.

This patch also refactors the code in ParsePragma.cpp a little bit to make it
more obvious that some codes are being shared between -fms-extensions and PS4.

llvm-svn: 233015