[test-suite] Set language level to c++14 to tests failing with gnu++17

The following tests fail to build on the AIX buildbot due to the change to set the
language level to gnu++17:

  test-suite :: MultiSource/Benchmarks/tramp3d-v4/tramp3d-v4.test
  test-suite :: SingleSource/Benchmarks/Misc-C++-EH/spirit.test
  test-suite :: SingleSource/Benchmarks/Misc-C++/stepanov_container.test

Reviewed By: daltenty, MaskRay

Differential Revision: https://reviews.llvm.org/D133591
3 files changed