[diagtool] Add a 'find-diagnostic-id' subcommand that converts a name of

the diagnostic to its enum value

This will be used by a script that invokes clang in a debugger and forces it
to stop when it reports a particular diagnostic.

Differential Revision: https://reviews.llvm.org/D35306


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@307813 91177308-0d34-0410-b5e6-96231b3b80d8