Explicitly set C++ standard for `Posix/no-fd.cc` ASan test.

Summary:
The test uses `nullptr` which can break running the test if the
compiler happens to be using something older than C++11 as the default
language standard. Avoid this by explicitly setting the standard.

rdar://problem/47253542

Reviewers: eugenis, yln, vitalybuka

Subscribers: kubamracek, #sanitizers, llvm-commits

Differential Revision: https://reviews.llvm.org/D56667

llvm-svn: 351169