[Tests] Add explicit -std=lang option to a number of tests.

This patch should not change the test results, but it is useful if clang's
default C++ language is ever changed from gnu++98.

Patch by: Charles Li

http://lists.cs.uiuc.edu/pipermail/cfe-commits/Week-of-Mon-20150727/134667.html

llvm-svn: 243819