Add Rust to CodeView SourceLanguage (CV_CFL_LANG) enum

Microsoft has added several new entries to the CV_CFL_LANG enum, including Rust:
    https://docs.microsoft.com/en-us/visualstudio/debugger/debug-interface-access/cv-cfl-lang

This change adds Rust to the corresponding LLVM enum and translates `dwarf::DW_LANG_Rust` to `SourceLanguage::Rust` in the CodeView AsmPrinter.

This means that Rust will no longer emit as Masm.

Differential Revision: https://reviews.llvm.org/D115300