Verifier: Check debug info intrinsic arguments

Verify that debug info intrinsic arguments are valid.  (These checks
will not recurse through the full debug info graph, so they don't need
to be cordoned of in `DebugInfoVerifier`.)

With those checks in place, changing the `DbgIntrinsicInst` accessors to
downcast to `MDLocalVariable` and `MDExpression` is natural (added isa
specializations in `Metadata.h` to support this).

Added tests to `test/Verifier` for the new -verify checks, and fixed the
debug info in all the in-tree tests.

If you have out-of-tree testcases that have started to fail to -verify,
hopefully the verify checks are helpful.  The most likely problem is
that the expression argument is `!{}` (instead of `!MDExpression()`).

llvm-svn: 232296