Add basic fix-its to SMDiagnostic.

Add basic fix-its to SMDiagnostic.

Like Clang's FixItHint, SMFixIt represents an insertion, replacement, or
removal of source text. One or more fix-its can be emitted as part of
a diagnostic, and will be printed below the source range line to show the
user how they can fix their code.

Currently, the only client of SMFixIt is clang-tblgen; thus, the tests for
this behavior live in clang/test/TableGen/tg-fixits.td. If/when SMFixIt is
adopted within LLVM itself, those tests should be moved to the LLVM suite.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@172086 91177308-0d34-0410-b5e6-96231b3b80d8