TableGen: record anonymous instantiations of classes.
TableGen: Keep track of superclass reference ranges.

def foo : bar;
          ~~~

This allows us to produce more precise diagnostics about a certain
superclass, and even provide fixits.
Add basic fix-its to SMDiagnostic.

Like Clang's FixItHint, SMFixIt represents an insertion, replacement, or
removal of source text. One or more fix-its can be emitted as part of
a diagnostic, and will be printed below the source range line to show the
user how they can fix their code.

Currently, the only client of SMFixIt is clang-tblgen; thus, the tests for
this behavior live in clang/test/TableGen/tg-fixits.td. If/when SMFixIt is
adopted within LLVM itself, those tests should be moved to the LLVM suite.
Error if an anonymous DiagGroup is referenced multiple times.

Not only is this inefficient for TableGen, it's annoying for maintenance
when renaming warning flags (unusual) or adding those flags to a group
(more likely).

This uses the new fix-it infrastructure for LLVM's SourceMgr/SMDiagnostic,
as well as a few changes to TableGen to track more source information.
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