[libc] Compile with -Wconversion -Wno-sign-conversion

This more closely matches the stricter warnings used for
this same code in the Fuchsia build.

Reviewed By: michaelrj

Differential Revision: https://reviews.llvm.org/D156630

libc/cmake/modules/LLVMLibCObjectRules.cmake

@@ -30,6 +30,8 @@ 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 "-Wconversion")
+    list(APPEND compile_options "-Wno-sign-conversion")
     list(APPEND compile_options "-Wimplicit-fallthrough")
     list(APPEND compile_options "-Wwrite-strings")
     list(APPEND compile_options "-Wextra-semi")