[Frontend] Handle skipped bodies in template instantiations

[Frontend] Handle skipped bodies in template instantiations

Summary:
- Fixed an assert in Sema::InstantiateFunctionDefinition and added
  support for instantiating a function template with skipped body.
- Properly call setHasSkippedBody for FunctionTemplateDecl passed to
  Sema::ActOnSkippedFunctionBody.

Reviewers: sepavloff, bkramer

Reviewed By: sepavloff

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D41237

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@321174 91177308-0d34-0410-b5e6-96231b3b80d8