Add driver flags -ftrigraphs, -fno-trigraphs.

-trigraphs is now an alias for -ftrigraphs.  -fno-trigraphs makes it possible
to explicitly disable trigraphs, which couldn't be done before.

  clang -std=c++11 -fno-trigraphs

now builds without GNU extensions, but with trigraphs disabled.  Previously,
trigraphs were only disabled in GNU modes or with -std=c++1z.

Make the new -f flags the cc1 interface too.  This requires changing -trigraphs
to -ftrigraphs in a few cc1 tests.

Related to PR21974.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@224790 91177308-0d34-0410-b5e6-96231b3b80d8