[compiler-rt] cmake: add include(BuiltinTests) to CompilerRTDarwinUtils

In cmake, if TEST_COMPILE_ONLY is set
compiler-rt/cmake/Modules/CompilerRTDarwinUtils.cmake invokes try_compile_only()
but try_compile_only() is defined in BuiltinTests.cmake and is not included in
CompilerRTDarwinUtils.cmake. This patch simply includes it BuiltinTests.

Differential Revision: https://reviews.llvm.org/D69410

Author: plotfi

compiler-rt/cmake/Modules/CompilerRTDarwinUtils.cmake

@@ -1,5 +1,6 @@
 include(CMakeParseArguments)
 include(CompilerRTUtils)
+include(BuiltinTests)
 
 set(CMAKE_LIPO "lipo" CACHE PATH "path to the lipo tool")