Auto merge of #89917 - davidtwco:issue-60705-stabilize-rust-symbol-mangling-scheme, r=wesleywiser

sess: default to v0 symbol mangling on nightly

cc rust-lang/rust#60705 rust-lang/compiler-team#938

Rust's current mangling scheme depends on compiler internals; loses information about generic parameters (and other things) which makes for a worse experience when using external tools that need to interact with Rust symbol names; is inconsistent; and can contain `.` characters which aren't universally supported. Therefore, Rust has defined its own symbol mangling scheme which is defined in terms of the Rust language, not the compiler implementation; encodes information about generic parameters in a reversible way; has a consistent definition; and generates symbols that only use the characters `A-Z`, `a-z`, `0-9`, and `_`.

Support for the new Rust symbol mangling scheme has been added to upstream tools that will need to interact with Rust symbols (e.g. debuggers).

This pull request changes the default symbol mangling scheme from the legacy scheme to the new Rust mangling scheme on nightly.

The following pull requests implemented v0 mangling in rustc (if I'm missing any, let me know):

- rust-lang/rust#57967
- rust-lang/rust#63559
- rust-lang/rust#75675
- rust-lang/rust#77452
- rust-lang/rust#77554
- rust-lang/rust#83767
- rust-lang/rust#87194
- rust-lang/rust#87789

Rust's symbol mangling scheme has support in the following external tools:

- `binutils`/`gdb` (GNU `libiberty`)
    - [[PATCH] Move rust_{is_mangled,demangle_sym} to a private libiberty header.
](https://gcc.gnu.org/pipermail/gcc-patches/2019-June/523011.html) committed as gcc-mirror/gcc@979526c
    - [[PATCH] Simplify and generalize rust-demangle's unescaping logic.
](https://gcc.gnu.org/pipermail/gcc-patches/2019-August/527835.html) committed as gcc-mirror/gcc@42bf58b
    - [[PATCH] Remove some restrictions from rust-demangle.
](https://gcc.gnu.org/pipermail/gcc-patches/2019-September/530445.html) committed as gcc-mirror/gcc@e1cb00d
    - [[PATCH] Refactor rust-demangle to be independent of C++ demangling.
](https://gcc.gnu.org/pipermail/gcc-patches/2019-November/533719.html) ([original submission](https://gcc.gnu.org/pipermail/gcc-patches/2019-October/532388.html)) committed as gcc-mirror/gcc@32fc371
    - [[PATCH] Support the new ("v0") mangling scheme in rust-demangle.
](https://gcc.gnu.org/pipermail/gcc-patches/2020-November/558905.html) ([original submission](https://gcc.gnu.org/pipermail/gcc-patches/2020-March/542012.html)) committed as gcc-mirror/gcc@8409649
- `lldb`/`llvm-objdump`/`llvm-nm`/`llvm-symbolizer`/`llvm-cxxfilt`/etc
  - llvm/llvm-project@7310403
  - llvm/llvm-project@c8c2b46
  - llvm/llvm-project@0a2d4f3
- Linux `perf`
- `valgrind`
  - [Update demangler to support Rust v0 name mangling.](https://bugs.kde.org/show_bug.cgi?id=431306)

rust-lang/rust#85530 (comment) contains a summary of the most recent crater run of the v0 mangling, and the remaining issues from that were fixed by rust-lang/rust#87194 (confirmed by follow-up crater run, rust-lang/rust#85530 (comment)).

`@rustbot` label +T-compiler
r? `@michaelwoerister`

Author: bors