[compiler-rt] Use COMPILER_RT_TEST_CXX_COMPILER for linking compiler-rt tests

[leonardchan]

Summary

Before, COMPILER_RT_TEST_COMPILER was used which pointed to a C compiler. While it is incorrect to assume either of these is the default compiler, using the C++ one allows for linking cpp tests.

[phosek]

LGTM

compiler-rt/cmake/Modules/AddCompilerRT.cmake

function(add_compiler_rt_test test_suite test_name arch)
   endif()
   if(NOT COMPILER_RT_STANDALONE_BUILD AND COMPILER_RT_HAS_LLD AND "lld" IN_LIST LLVM_ENABLE_PROJECTS)
     # CMAKE_EXE_LINKER_FLAGS may contain -fuse=lld
     # FIXME: -DLLVM_ENABLE_LLD=ON and -DLLVM_ENABLE_PROJECTS without lld case.
     list(APPEND TEST_DEPS lld)
   endif()
   add_custom_command(
     OUTPUT "${output_bin}"
-    COMMAND ${COMPILER_RT_TEST_COMPILER} ${TEST_OBJECTS} -o "${output_bin}"
+    COMMAND ${COMPILER_RT_TEST_CXX_COMPILER} ${TEST_OBJECTS} -o "${output_bin}"
             ${TEST_LINK_FLAGS}
     DEPENDS ${TEST_DEPS}
     )
   add_custom_target(T${test_name} DEPENDS "${output_bin}")
   set_target_properties(T${test_name} PROPERTIES FOLDER "Compiler-RT Tests")
 
   # Make the test suite depend on the binary.
   add_dependencies(${test_suite} T${test_name})