Diagnostics: name all implicit groups used more than once.

This will be a new style requirement going forwards: a diagnostic can only
use the implicit InGroup<DiagGroup<"foo">> syntax if "foo" is not used by
any other diagnostics; as soon as it is, it needs an explicit group.

This also brings some stray "conversion" diagnostics into the
"Value Conversion Issue" category, instead of the more generic
"Semantic Issue" category. I consider this an improvement!
- warn_impcast_complex_scalar
- warn_impcast_float_integer
- warn_impcast_float_precision
- warn_impcast_integer_precision
- warn_impcast_vector_scalar

llvm-svn: 172088