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.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173746 91177308-0d34-0410-b5e6-96231b3b80d8