Pass -Werror when building LLVM

Enabling warning_into_errors() so deprecated uses of LLVM methods
can be treated as errors.

Author: rperier

compiler/rustc_llvm/build.rs

@@ -167,6 +167,7 @@ fn main() {
     let cxxflags = output(&mut cmd);
     let mut cfg = cc::Build::new();
     cfg.warnings(false);
+    cfg.warnings_into_errors(true);
     for flag in cxxflags.split_whitespace() {
         // Ignore flags like `-m64` when we're doing a cross build
         if is_crossed && flag.starts_with("-m") {