ASTTests/DeclTest.cpp: Tweak on msvc target to add "-fno-ms-extensions".

MS extensions don't prefer imaginary suffix even with -std=c++11.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@183301 91177308-0d34-0410-b5e6-96231b3b80d8