Switch C compilations to C11 by default.

This is long-since overdue, and matches GCC 5.0. This should also be
backwards-compatible, because we already supported all of C11 as an extension
in C99 mode.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@220244 91177308-0d34-0410-b5e6-96231b3b80d8