Refactor librustc_codegen_llvm for multiple codegen backends.

[eddyb]

[#50615] Rename librustc_trans to librustc_codegen_llvm.
Make rustc_driver either able to choose codegen backends, or even better, pass rustc_codegen_llvm to rustc_driver from rustc-main.
See also #45226.

Possible refactoring steps:

• remove SharedCrateContext, use TyCtxt
• rename (Local)CrateContext to CodegenContext, remove accessor methods
• [ rustc_codegen_llvm: use safe references for LLVM FFI types. #52461] merge librustc_llvm into librustc_codegen_llvm
  • pass ValueRef as &'ll Value instead, encode LLVM lifetimes in FFI signatures
  • use this technique for C++ objects that borrow others: Tracking issue for RFC 1861: Extern types #43467 (comment)
• use this technique for the DI* inheritance hierarchy: Tracking issue for RFC 1861: Extern types #43467 (comment)
• use &'ll Module<'ll>, &'ll Value<'ll>, &'ll Type<'ll> etc. to correctly model lifetime relationships (without an invariant lifetime parameter, very little is actually enforced)
• refer to cranelift and llvm2cranelift for common Cranelift/LLVM builder APIs
  • [rustc_codegen_llvm: traitification of LLVM-specific CodegenCx and Builder methods #55627] common tools used by SSA backends could be placed in rustc_codegen_ssa
  • refactor rustc_codegen_ssa APIs to match Cranelift, leave LLVM second-class
    • unaddressed comments from rustc_codegen_llvm: traitification of LLVM-specific CodegenCx and Builder methods #55627 may be relevant here
• split the metadata, "optimize + generate machine code" (should it be called "binemit"?) and linking components, out of rustc_codegen_ssa