Split out types that are non-canonical unless dependent as their own

category.  Use this in a few places to eliminate unnecessary TST cases and
do some future-proofing.  Provide terrible manglings for typeof.  Mangle
decltype with some hope of accuracy.

Our manglings for some of the cases covered in the testcase are different
from gcc's, which I've raised as an issue with the ABI list.

llvm-svn: 97523