[ASTImporter] import FunctionDecl end locations

Summary: On constructors that do not take the end source location, it was not imported. Fixes test from D47698 / rC336269.

Reviewers: martong, a.sidorin, balazske, xazax.hun, a_sidorin

Reviewed By: martong, a_sidorin

Subscribers: a_sidorin, rnkovacs, cfe-commits

Differential Revision: https://reviews.llvm.org/D48941

llvm-svn: 336523