add LLVM_LIBC_WNO_ERROR config

Author: nickdesaulniers

libc/cmake/modules/LLVMLibCObjectRules.cmake

@@ -43,7 +43,10 @@ function(_get_common_compile_options output_var flags)
     list(APPEND compile_options "-fno-rtti")
     list(APPEND compile_options "-Wall")
     list(APPEND compile_options "-Wextra")
-    list(APPEND compile_options "-Werror")
+    # -DLLVM_LIBC_WNO_ERROR=ON if you can't build cleanly with -Werror.
+    if(NOT LLVM_LIBC_WNO_ERROR)
+      list(APPEND compile_options "-Werror")
+    endif()
     list(APPEND compile_options "-Wconversion")
     list(APPEND compile_options "-Wno-sign-conversion")
     list(APPEND compile_options "-Wimplicit-fallthrough")