Don't demangle Rust symbols by default, but allow linking to rust-demangle.

The Rust compiler uses GCC C++ name mangling, but it has another layer of
encoding so abi::cxa_demangle doesn't produce great results. This patch
changes dump_syms to dump unmangled names by default so that consumers can
demangle them after-the-fact.

It also adds a tiny bit of support for linking against a Rust library I wrote
that can demangle Rust symbols nicely:
https://github.com/luser/rust-demangle-capi

BUG=

Change-Id: I63a425035ebb7ac516f067fed2aa782849ea9604
Reviewed-on: https://chromium-review.googlesource.com/402308
Reviewed-by: Mark Mentovai <mark@chromium.org>

src/common/dwarf_cu_to_module.cc

@@ -777,6 +777,10 @@ void DwarfCUToModule::SetLanguage(DwarfLanguage language) {
       cu_context_->language = Language::Swift;
       break;
 
+    case dwarf2reader::DW_LANG_Rust:
+      cu_context_->language = Language::Rust;
+      break;
+
     // DWARF has no generic language code for assembly language; this is
     // what the GNU toolchain uses.
     case dwarf2reader::DW_LANG_Mips_Assembler: