[Sema] Fix a pair of crashes when generating exception specifiers with an

error'ed field for a template class' default ctor.

The two examples in the test would both cause a compiler assert when attempting
to calculate the exception specifier for the default constructor for the
template classes. The problem was that dependents of this function expect that
Field->getInClassInitializer (including canThrow) is not nullptr. However, if
the template's initializer has an error, exactly that situation happens.

This patch simply sets the field to be invalid.

Differential Revision: https://reviews.llvm.org/D37865

llvm-svn: 313569