PR21437, final part of DR1330: delay-parsing of exception-specifications. This
is a re-commit of Doug's r154844 (modernized and updated to fit into current
Clang).


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@221918 91177308-0d34-0410-b5e6-96231b3b80d8