[MS Extensions] Remove support for the i128 integer literal suffix

There is currently no support in MSVC for using i128 as an integer
literal suffix.  In fact, there appears to be no evidence that they have
ever supported this feature in any of their compilers.  This was an over
generalization of their actual feature and is a nasty source of bugs.
Why is it a source of bugs?  Because most code in clang expects that
evaluation of an integer constant expression won't give them something
that 'long long' can't represent.  Instead of providing a meaningful
feature, i128 gives us cute ways of exploding the compiler.

llvm-svn: 243243