[compiler-rt] Fix C and C++ compilers being switched when compiling tests (#110552)

The logic was simply switched. Fixes building tests for me.  
Not sure how I haven't stumbled upon this before considering this code hasn't changed lately and my setup's been the same.

Fixes: 07317bb

Author: tambry

compiler-rt/cmake/Modules/CompilerRTCompile.cmake

@@ -73,9 +73,9 @@ function(clang_compile object_file source)
   endif()
   string(REGEX MATCH "[.](cc|cpp)$" is_cxx ${source_rpath})
   if (is_cxx)
-    set(compiler ${COMPILER_RT_TEST_COMPILER})
-  else()
     set(compiler ${COMPILER_RT_TEST_CXX_COMPILER})
+  else()
+    set(compiler ${COMPILER_RT_TEST_COMPILER})
   endif()
   if(COMPILER_RT_STANDALONE_BUILD)
     # Only add global flags in standalone build.