Thread Safety: Added basic argument parsing for all new attributes.

This patch special cases the parser for thread safety attributes so that all
attribute arguments are put in the argument list (instead of a special
parameter) since arguments may not otherwise resolve correctly without two-token
lookahead.

This patch also adds checks to make sure that attribute arguments are
lockable objects.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@137130 91177308-0d34-0410-b5e6-96231b3b80d8