[Driver] Add -fno-digraphs

Summary: Add a flag `-fno-digraphs` to disable digraphs in the lexer, similar to `-fno-operator-names` which disables alternative names for C++ operators.

Reviewers: rsmith

Reviewed By: rsmith

Subscribers: rsmith, cfe-commits

Differential Revision: https://reviews.llvm.org/D48266

llvm-svn: 337232