Perform stricter/more accurate checking for C99 constant expressions

Perform stricter/more accurate checking for C99 constant expressions
in Sema, per discussion on mailing list.  This doesn't cause any changes
in the test results.  I'll probably add some more tests at some point,
but it's an open question what we need to accept to be compatible with
real code.

This doesn't touch the existing isConstantExpr method on Expr; that
should be addressed somehow eventually (either removed or refined to
whatever is appropriate).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@51318 91177308-0d34-0410-b5e6-96231b3b80d8