rust/src/rustllvm

This directory currently contains some LLVM support code. This will generally
be sent upstream to LLVM in time; for now it lives here.

NOTE: the LLVM C++ ABI is subject to between-version breakage and must *never*
be exposed to Rust. To allow for easy auditing of that, all Rust-exposed types
must be typedef-ed as "LLVMXyz", or "LLVMRustXyz" if they were defined here.

Functions that return a failure status and leave the error in
the LLVM last error should return an LLVMRustResult rather than an
int or anything to avoid confusion.

When translating enums, add a single `Other` variant as the first
one to allow for new variants to be added. It should abort when used
as an input.

All other types must not be typedef-ed as such.