Move -Wkeyword-macro into -pedantic, remove -Wreserved-id-macro.

As discussed on the post-commit review thread for r224012, -Wkeyword-macro fires
mostly on headers trying to set up portable defines and doesn't find much bad
stuff in practice.  But [macro.names]p2 does disallow defining or undefining
keywords, override and final, and alignas, so keep the warning but move it
into -pedantic.

-Wreserved-id-macro warns on

    #define __need_size_t

which is more or less public api for glibc headers. Since this warning isn't
motivated by a standard, remove it.

(See also r223114 for a previous follow-up to r224012.)

llvm-svn: 224371