[CMake] Build lib/gwp_asan/tests with -std=c++17

[MaskRay]

Summary

With the recent llvm-project C++17 switch (D130689), gwp_asan/tests may fail to
link with some versions of GCC (https://github.com/llvm/llvm-project/issues/56994):

backtrace.cpp:(.text+0xca6): undefined reference to `gwp_asan::AllocationMetadata::kMaxTraceLengthToCollect'

I cannot reproduce this issue by myself, but notice that currently
lib/gwp_asan/*.cpp get -std=c++17 while lib/gwp_asan/tests/*.cpp don't
(therefore may use -std=g++14 default from Clang and older GCC). Using -std=c++17
for lib/gwp_asan/tests will ensure that backtrace.cpp uses inline variable and will assuredly avoid the
possible GCC issue.

In the long-term, we should add -std=c++17 to a central place like generate_compiler_rt_tests.

[dyung]

LGTM. The change fixes the problem on my local machine. Thanks!

[MaskRay]

Landing this to fix the build bot issue. A dozen *_UNITTEST_CFLAGS lack -std=c++17 but I do not find a good place to add the option...

compiler-rt/lib/gwp_asan/tests/CMakeLists.txt

 include(CompilerRTCompile)
 
 set(GWP_ASAN_UNITTEST_CFLAGS
   ${COMPILER_RT_UNITTEST_CFLAGS}
   ${COMPILER_RT_GTEST_CFLAGS}
   ${SANITIZER_TEST_CXX_CFLAGS}
+  -std=c++17
   -I${COMPILER_RT_SOURCE_DIR}/lib/
   -O2
   -g
   -fno-omit-frame-pointer)
 
 file(GLOB GWP_ASAN_HEADERS ../*.h)
 set(GWP_ASAN_UNITTESTS
   platform_specific/printf_sanitizer_common.cpp