Fix typo in llvm-config.h.cmake

This patch fixes a typo in https://reviews.llvm.org/rGa1862d78eb45 where
parenthesis was placed by in place of curly braces which was strangely
building with 3.17.

Differential Revision: https://reviews.llvm.org/D114639

llvm/include/llvm/Config/llvm-config.h.cmake

@@ -110,6 +110,6 @@
 #cmakedefine LLVM_BUILD_SHARED_LIBS
 
 /* Define if building LLVM with LLVM_FORCE_USE_OLD_TOOLCHAIN_LIBS */
-#cmakedefine LLVM_FORCE_USE_OLD_TOOLCHAIN $(LLVM_FORCE_USE_OLD_TOOLCHAIN)
+#cmakedefine LLVM_FORCE_USE_OLD_TOOLCHAIN ${LLVM_FORCE_USE_OLD_TOOLCHAIN}
 
 #endif