Replace AS_MSTypespec with AS_Keyword, for representing any attribute spelled

Replace AS_MSTypespec with AS_Keyword, for representing any attribute spelled
as a keyword. Rationalize existing attributes to use it as appropriate, and to
not lie about some __declspec attributes being GNU attributes. In passing,
remove a gross hack which was discarding attributes which we could handle. This
results in us actually respecting the __pascal keyword again.

llvm-svn: 173746