Check for an invalid field earlier in a constructor's initialization

Check for an invalid field earlier in a constructor's initialization
of that field. Otherwise, we can end up building and later trying to
instantiate a dependent member initializer that will fail at
instantiation time.

Unfortunately, I've only managed to trigger this bug with very large
sources, so there's no test case :(

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@118306 91177308-0d34-0410-b5e6-96231b3b80d8