Revert "Revert " [runtime] unittests should use just-built compiler if the ru…"

Author: aschwaighofer

unittests/runtime/CMakeLists.txt

@@ -2,7 +2,10 @@ if(("${SWIFT_HOST_VARIANT_SDK}" STREQUAL "${SWIFT_PRIMARY_VARIANT_SDK}") AND
    ("${SWIFT_HOST_VARIANT_ARCH}" STREQUAL "${SWIFT_PRIMARY_VARIANT_ARCH}"))
 
   if("${CMAKE_C_COMPILER_ID}" MATCHES "Clang")
-    # Do nothing
+    if(NOT SWIFT_BUILD_RUNTIME_WITH_HOST_COMPILER)
+      set(CMAKE_CXX_COMPILER "${SWIFT_NATIVE_CLANG_TOOLS_PATH}/clang++")
+      set(CMAKE_C_COMPILER "${SWIFT_NATIVE_CLANG_TOOLS_PATH}/clang")
+    endif()
   elseif(SWIFT_BUILD_RUNTIME_WITH_HOST_COMPILER)
     if(NOT "${CMAKE_C_COMPILER_ID}" MATCHES "Clang")
       message(FATAL_ERROR "Building the swift runtime is not supported with ${CMAKE_C_COMPILER_ID}. Use the just-built clang instead.")