[diagtool] Add 'show-enabled', which displays which warnings are enabled.

show-enabled uses the command line you give it to build a CompilerInstance,
so any flags you pass will be processed as if running clang proper.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@157842 91177308-0d34-0410-b5e6-96231b3b80d8
7 files changed