Add CompilerInstance::createDiagnostics, and move clang-cc to it.

clang-cc.cpp is now under 1k lines, if anyone is counting.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@87090 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed