[cmake] Fix monorepo + LLVM_BUILD_EXTERNAL_COMPILER_RT=ON.

In cmake 3.10.2, if (${VARIABLE_NAME}) seems to always be false no
matter what documentation says (or maybe I just failed at reading).

Anyway, if (VARIABLE_NAME) seems to do what this code intended.

llvm-svn: 350361

Author: eugenis

clang/runtime/CMakeLists.txt

@@ -28,7 +28,7 @@ set(COMPILER_RT_SRC_ROOT ${LLVM_MAIN_SRC_DIR}/projects/compiler-rt)
 # variable) as in add_llvm_external_project
 if(NOT EXISTS ${COMPILER_RT_SRC_ROOT})
   # We don't want to set it if LLVM_EXTERNAL_COMPILER_RT_SOURCE_DIR is ""
-  if(${LLVM_EXTERNAL_COMPILER_RT_SOURCE_DIR})
+  if(LLVM_EXTERNAL_COMPILER_RT_SOURCE_DIR)
     set(COMPILER_RT_SRC_ROOT ${LLVM_EXTERNAL_COMPILER_RT_SOURCE_DIR})
   endif()
 endif()