Rust frontend patches v3

[arthur.cohen@embecosm.com]

This is the fixed version of our previous patch set for gccrs - We've adressed
the comments raised in our previous emails.

This patch set does not contain any work that was not previously included, such
as closure support, the constant evaluator port, or the better implementation
of target hooks by Iain Buclaw. They will follow up in subsequent patch sets.

Thanks again to Open Source Security, inc and Embecosm who have accompanied us
for this work.

Many thanks to all of the contributors and our community, who made this
possible.

A very special thanks to Philip Herron, without whose mentoring I would have
never been in a position to send these patches.

You can see the current status of our work on our branch:
https://gcc.gnu.org/git/?p=gcc.git;a=shortlog;h=refs/heads/devel/rust/master

The patch set contains the following:

[PATCH Rust front-end v3 01/46] Use DW_ATE_UTF for the Rust 'char'
[PATCH Rust front-end v3 02/46] gccrs: Add nessecary hooks for a Rust
[PATCH Rust front-end v3 03/46] gccrs: Add Debug info testsuite
[PATCH Rust front-end v3 04/46] gccrs: Add link cases testsuite
[PATCH Rust front-end v3 05/46] gccrs: Add general compilation test
[PATCH Rust front-end v3 06/46] gccrs: Add execution test cases
[PATCH Rust front-end v3 07/46] gccrs: Add gcc-check-target
[PATCH Rust front-end v3 08/46] gccrs: Add Rust front-end base AST
[PATCH Rust front-end v3 09/46] gccrs: Add definitions of Rust Items
[PATCH Rust front-end v3 10/46] gccrs: Add full definitions of Rust
[PATCH Rust front-end v3 11/46] gccrs: Add Rust AST visitors
[PATCH Rust front-end v3 12/46] gccrs: Add Lexer for Rust front-end
[PATCH Rust front-end v3 13/46] gccrs: Add Parser for Rust front-end
[PATCH Rust front-end v3 14/46] gccrs: Add Parser for Rust front-end
[PATCH Rust front-end v3 15/46] gccrs: Add expansion pass for the
[PATCH Rust front-end v3 16/46] gccrs: Add name resolution pass to
[PATCH Rust front-end v3 17/46] gccrs: Add declarations for Rust HIR
[PATCH Rust front-end v3 18/46] gccrs: Add HIR definitions and
[PATCH Rust front-end v3 19/46] gccrs: Add AST to HIR lowering pass
[PATCH Rust front-end v3 20/46] gccrs: Add wrapper for make_unique
[PATCH Rust front-end v3 21/46] gccrs: Add port of FNV hash used
[PATCH Rust front-end v3 22/46] gccrs: Add Rust ABI enum helpers
[PATCH Rust front-end v3 23/46] gccrs: Add Base62 implementation
[PATCH Rust front-end v3 24/46] gccrs: Add implementation of Optional
[PATCH Rust front-end v3 25/46] gccrs: Add attributes checker
[PATCH Rust front-end v3 26/46] gccrs: Add helpers mappings canonical
[PATCH Rust front-end v3 27/46] gccrs: Add type resolution and trait
[PATCH Rust front-end v3 28/46] gccrs: Add Rust type information
[PATCH Rust front-end v3 29/46] gccrs: Add remaining type system
[PATCH Rust front-end v3 30/46] gccrs: Add unsafe checks for Rust
[PATCH Rust front-end v3 31/46] gccrs: Add const checker
[PATCH Rust front-end v3 32/46] gccrs: Add privacy checks
[PATCH Rust front-end v3 33/46] gccrs: Add dead code scan on HIR
[PATCH Rust front-end v3 34/46] gccrs: Add unused variable scan
[PATCH Rust front-end v3 35/46] gccrs: Add metadata ouptput pass
[PATCH Rust front-end v3 36/46] gccrs: Add base for HIR to GCC
[PATCH Rust front-end v3 37/46] gccrs: Add HIR to GCC GENERIC
[PATCH Rust front-end v3 38/46] gccrs: Add HIR to GCC GENERIC
[PATCH Rust front-end v3 39/46] gccrs: These are wrappers ported from
[PATCH Rust front-end v3 40/46] gccrs: Add GCC Rust front-end
[PATCH Rust front-end v3 41/46] gccrs: Add config-lang.in
[PATCH Rust front-end v3 42/46] gccrs: Add lang-spec.h
[PATCH Rust front-end v3 43/46] gccrs: Add lang.opt
[PATCH Rust front-end v3 44/46] gccrs: Add compiler driver
[PATCH Rust front-end v3 45/46] gccrs: Compiler proper interface
[PATCH Rust front-end v3 46/46] gccrs: Add README, CONTRIBUTING and