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

[ZarkoCA]

Summary

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

[MaskRay]

LG. The 3 programs build on a Debian x86-64 machine. Is the build failure AIX specific due to AIX-specific code or PowerPC-specific code in the benchmarks?

[daltenty]

These benchmarks use features removed in C++17: https://lab.llvm.org/buildbot/#/builders/214/builds/3211/steps/9/logs/stdio
Suspect this is broken for other libc++ based platforms.

MultiSource/Benchmarks/tramp3d-v4/CMakeLists.txt

+set(CMAKE_CXX_STANDARD 14)
 list(APPEND CXXFLAGS -fno-exceptions)
 if(SMALL_PROBLEM_SIZE)
   set(RUN_OPTIONS --cartvis 1.0 0.0 --rhomin 1e-8 -n 4 --domain 32 32 32)
 else()
   set(RUN_OPTIONS --cartvis 1.0 0.0 --rhomin 1e-8 -n 10 --domain 32 32 32)
 endif()
 if(MIPS_IS_MIPS64_ENABLED)
   list(APPEND CXXFLAGS -mxgot)
 endif()
 set(FP_ABSTOLERANCE 0.0000001)
 llvm_multisource(tramp3d-v4)

SingleSource/Benchmarks/Misc-C++-EH/CMakeLists.txt

+set(CMAKE_CXX_STANDARD 14)
 list(APPEND CXXFLAGS -pthread)
 list(APPEND LDFLAGS -lm -pthread)
 llvm_singlesource()

SingleSource/Benchmarks/Misc-C++/CMakeLists.txt

+set(CMAKE_CXX_STANDARD 14)
 add_subdirectory(Large)
 
 list(APPEND LDFLAGS -lm)
 set(FP_ABSTOLERANCE 0.01)
 llvm_singlesource()