add the ability to associate 'category' names with clang diagnostics

add the ability to associate 'category' names with clang diagnostics
and diagnostic groups.  This allows the compiler to group
diagnostics together (e.g. "Logic Warning",
"Format String Warning", etc) like the static analyzer does.
This is not exposed through anything in the compiler yet.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@103050 91177308-0d34-0410-b5e6-96231b3b80d8