Proper dependency check for clang in compiler_rt.

- Not having a dependency does not work in standalone build, as Clang does not exist.
 - if (TARGET clang) check is useless, as it is order-dependent,
   and Clang may not be registered yet.

Differential Revision: https://reviews.llvm.org/D37228

llvm-svn: 311911

compiler-rt/cmake/Modules/CompilerRTCompile.cmake

@@ -136,7 +136,7 @@ macro(clang_compiler_add_cxx_check)
       COMMAND bash -c "${CMD}"
       COMMENT "Checking that just-built clang can find C++ headers..."
       VERBATIM)
-    if (TARGET clang)
+    if (NOT COMPILER_RT_STANDALONE_BUILD)
       ADD_DEPENDENCIES(CompilerRTUnitTestCheckCxx clang)
     endif()
   endif()