Driver: Add a Tool::hasGoodDiagnostics hook, and use it to simplify logic for

Driver: Add a Tool::hasGoodDiagnostics hook, and use it to simplify logic for
deciding when we need to emit an extra "command failed" diagnostic.
 - This also fixes the case where we were emitting that extra diagnostics, even
   when using clang w/ the integrated assembler, which has good diagnostics.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@100529 91177308-0d34-0410-b5e6-96231b3b80d8