[libc] build with -Werror

A recent commit introduced warnings observable when building unit tests. If the
unit tests don't fail when warnings are introduced into the build, then we
might fail to notice them in the stream of output from check-libc.

Link: https://github.com/llvm/llvm-project/pull/72763/files#r1410932348

Author: nickdesaulniers

libc/cmake/modules/LLVMLibCObjectRules.cmake

@@ -43,6 +43,7 @@ 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")
     list(APPEND compile_options "-Wconversion")
     list(APPEND compile_options "-Wno-sign-conversion")
     list(APPEND compile_options "-Wimplicit-fallthrough")