Per offline discussion with Steve Naroff, add back Type::getAsXXXType() methods

until Doug Gregor's Type smart pointer code lands (or more discussion occurs).
These methods just call the new Type::getAs<XXX> methods, so we still have
reduced implementation redundancy. Having explicit getAsXXXType() methods makes
it easier to set breakpoints in the debugger.

llvm-svn: 76193