[Frontend] Correctly handle instantiating ctors with skipped bodies

Summary:
Previsouly clang tried instantiating member initializers even if ctor
body was skipped, this caused spurious errors (see the test).

Reviewers: sepavloff, klimek

Reviewed By: sepavloff

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D41492

llvm-svn: 321520