Add regression test for expressions calling functions taking anonymous struct typedef arguments

This CL adds a regression test for the bug listed at https://llvm.org/bugs/show_bug.cgi?id=26790
Functionality was implemented in commit r263544

Author: Luke Drummond <luke.drummond@codeplay.com>
Differential Revision: http://reviews.llvm.org/D17777

llvm-svn: 263547