Tests for "Disabling of "redefine_extname" pragma for C++ code"

In response to Richard Smith's comment (http://lists.cs.uiuc.edu/pipermail/cfe-commits/Week-of-Mon-20150622/131782.html), this patch disables "redefine_extname" pragma for C++ code. Also, I added a test that this pragma doesn't apply to static declarations.
Differential Revision: http://reviews.llvm.org/D10805

llvm-svn: 242507