When a declaration of a function is missing an exception specification

When a declaration of a function is missing an exception specification
that was present in a prior declaration, emit a warning rather than a
hard error (which we did before, and still do with mismatched
exception specifications). Moreover, provide a fix-it hint with the
throw() clause that should be added, e.g.,

t.C:10:7: warning: 'operator new' is missing exception specification
      'throw(std::bad_alloc)'
void *operator new(unsigned long sz)
      ^
                                     throw(std::bad_alloc)

As part of this, disable the warning when we're missing an exception
specification on operator new, operator new[], operator delete, or
operator delete[] when exceptions are turned off (-fno-exceptions).

Fixes PR5957.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@99388 91177308-0d34-0410-b5e6-96231b3b80d8