Use the new language identifier for Rust in the PDB debug format

Rust currently identifies as MASM (Microsoft Assembler) in the PDB
debug info format on Windows because no identifier was available.

This change pulls in a cherry-pick to Rust's LLVM that includes the
change to use the new identifier for Rust.

https://docs.microsoft.com/en-us/visualstudio/debugger/debug-interface-access/cv-cfl-lang