Make a test use a specific C++ dialect

In stack-reuse-miscompile.cpp, the allocas for the temps come out in
a different order depending on whether the dialect is C++03 or
C++11. Specify C++03 explicitly to avoid depending on the default.

Differential Revision: http://reviews.llvm.org/D27956


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@290208 91177308-0d34-0410-b5e6-96231b3b80d8