[libc] build with -Werror (#73966)

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 +43,10 @@ function(_get_common_compile_options output_var flags)
     list(APPEND compile_options "-fno-rtti")
     list(APPEND compile_options "-Wall")
     list(APPEND compile_options "-Wextra")
+    # -DLIBC_WNO_ERROR=ON if you can't build cleanly with -Werror.
+    if(NOT 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")

libc/docs/dev/code_style.rst

@@ -178,3 +178,11 @@ these functions do not call the constructors and destructors of the
 allocated/deallocated objects. So, use these functions carefully and only
 when it is absolutely clear that constructor and destructor invocation is
 not required.
+
+Warnings in sources
+===================
+
+We expect contributions to be free of warnings from the `minimum supported
+compiler versions`__ (and newer).
+
+.. __: https://libc.llvm.org/compiler_support.html#minimum-supported-versions