Documentation fix: remove advice to use cast/dyn_cast on TypeLocs, updating
it to refer to castAs/getAs.

The original change to remove the (broken) cast/dyn_cast support from TypeLoc
was in r175462.  (Thanks to David Blaikie for the reference.)


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@189908 91177308-0d34-0410-b5e6-96231b3b80d8