Promote the extension warning for attempts to catch a reference or

Promote the extension warning for attempts to catch a reference or
pointer to incomplete type from an ExtWarn to an error. We put the
ExtWarn in place as part of a workaround for Boost (PR6527), but it
(1) doesn't actually match a GCC extension and (2) has been fixed for
two years in Boost, and (3) causes us to emit code that fails badly at
run time, so it's a bad idea to keep it. Fixes PR11803.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@148838 91177308-0d34-0410-b5e6-96231b3b80d8