Merge #1811

1811: Merge upstream (dummy), "Rust front-end patches v4" r=tschwinge a=tschwinge

Last we did #1700 "Merge upstream, last commit before GCC/Rust upstreaming",
which merged commit b4fddbe9592e9feb37ce567d90af822b75995531 from GCC upstream
master branch.

This one now merges the "Rust front-end patches v4", commits
b4fddbe9592e9feb37ce567d90af822b75995531..edc676cfe8988c62c81b0df224c7fe82583012b1:

      - dd950cbbb97 Use DW_ATE_UTF for the Rust 'char' type
      - b772a504eff gccrs: Add necessary hooks for a Rust front-end testsuite
      - 4b8f3005af0 gccrs: Add Debug info testsuite
      - f60df7e6202 gccrs: Add link cases testsuite
      - 97705b4459b gccrs: Add general compilation test cases
      - 5a56869d6e3 gccrs: Add execution test cases
      - dc4171edb3c gccrs: Add gcc-check-target check-rust
      - 6b35ae12ce9 gccrs: Add Rust front-end base AST data structures
      - 438ae944fa6 gccrs: Add definitions of Rust Items in AST data structures
      - d588754c826 gccrs: Add full definitions of Rust AST data structures
      - 5b981e9c741 gccrs: Add Rust AST visitors
      - 18f6990f842 gccrs: Add Lexer for Rust front-end
      - 35e4f3b4af4 gccrs: Add Parser for Rust front-end pt.1
      - 32c8fb0eeaf gccrs: Add Parser for Rust front-end pt.2
      - 1841081a8a3 gccrs: Add expansion pass for the Rust front-end
      - 85a8fe00f80 gccrs: Add name resolution pass to the Rust front-end
      - 8ad1d56d68a gccrs: Add declarations for Rust HIR
      - 7641eaead40 gccrs: Add HIR definitions and visitor framework
      - 7999cf327de gccrs: Add AST to HIR lowering pass
      - b32b1b1576a gccrs: Add wrapper for make_unique
      - c7f8347e83c gccrs: Add port of FNV hash used during legacy symbol mangling
      - 15b0278905e gccrs: Add Rust ABI enum helpers
      - eb10bc5225e gccrs: Add Base62 implementation
      - 9a4fee5f57c gccrs: Add implementation of Optional
      - 2e7fc8780e0 gccrs: Add attributes checker
      - 9ce37e72062 gccrs: Add helpers mappings canonical path and lang items
      - c6c3db21769 gccrs: Add type resolution and trait solving pass
      - 24393cb68fa gccrs: Add Rust type information
      - 06688fe40a2 gccrs: Add remaining type system transformations
      - b1b35204d8a gccrs: Add unsafe checks for Rust
      - 5215235f016 gccrs: Add const checker
      - ca246e573fb gccrs: Add privacy checks
      - 520b52b24e7 gccrs: Add dead code scan on HIR
      - 4d67468d1d4 gccrs: Add unused variable scan
      - 509e4c32c6a gccrs: Add metadata output pass
      - 15f04af347e gccrs: Add base for HIR to GCC GENERIC lowering
      - 019b2f15581 gccrs: Add HIR to GCC GENERIC lowering for all nodes
      - cfbda2f78ba gccrs: Add HIR to GCC GENERIC lowering entry point
      - fe6264fa28a gccrs: These are wrappers ported from reusing gccgo
      - bba14a0790f gccrs: Add compiler driver
      - ea34614225d gccrs: Compiler proper interface kicks off the pipeline
      - ab1e0db43c2 gccrs: Add lang-spec.h
      - 5e7d199739f gccrs: Add lang.opt
      - 88415d33bb3 gccrs: Add GCC Rust front-end Make-lang.in
      - b07ef39ffbf gccrs: Add fatal_error when experimental flag is not present
      - a75f038c069 gccrs: Add config-lang.in
      - edc676cfe89 gccrs: Add README, CONTRIBUTING and compiler logo

This merge is done with `git merge --strategy=ours`, so that we effectively
don't bring any changes.  Rationale: any changes due to upstream review etc.,
have already been applied individually to GCC/Rust master branch, and any
remaining changes we'd either like to persist, or assess individually, later.

Co-authored-by: Tom Tromey <tom@tromey.com>
Co-authored-by: Philip Herron <philip.herron@embecosm.com>
Co-authored-by: Joel Phillips <simplytheother@gmail.com>
Co-authored-by: Arthur Cohen <arthur.cohen@embecosm.com>