Promote a warning on ill-formed code (redeclaration missing an exception

Promote a warning on ill-formed code (redeclaration missing an exception
specification) to an error. No compiler other than Clang seems to allow this,
and it doesn't seem like a useful thing to accept as an extension in general.

The current behavior was added for PR5957, where the problem was specifically
related to mismatches of the exception specification on the implicitly-declared
global operator new and delete. To retain that workaround, we downgrade the
error to an ExtWarn when the declaration is of a replaceable global allocation
function.

Now that this is an error, stop trying (and failing) to recover from a missing
computed noexcept specification. That recovery didn't work, and led to crashes
in code like the added testcase.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@248867 91177308-0d34-0410-b5e6-96231b3b80d8