Bump bootstrap compiler version to 1.55

[fee1-dead]

This PR is #87622 with the const_fn_union feature attributes readded, as it looks like it wasn't backported to beta.

r? @Mark-Simulacrum
cc @pietroalbini

(I don't have the permission to r= the PR)

[rust-highfive]

Some changes occurred in src/tools/clippy.

cc @rust-lang/clippy

[rust-log-analyzer]

The job x86_64-gnu-llvm-10 failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

   Compiling alloc v0.0.0 (/checkout/library/alloc)
   Compiling cfg-if v0.1.10
   Compiling adler v0.2.3
   Compiling rustc-demangle v0.1.18
error[E0658]: pattern bindings after an `@` are unstable
    |
    |
872 |         let src @ Range { start, end } = slice::range(src, ..self.len());
    |
    = note: see issue #65490 <https://github.com/rust-lang/rust/issues/65490> for more information
    = help: add `#![feature(bindings_after_at)]` to the crate attributes to enable


error[E0658]: pattern bindings after an `@` are unstable
    |
    |
872 |         let src @ Range { start, end } = slice::range(src, ..self.len());
    |
    = note: see issue #65490 <https://github.com/rust-lang/rust/issues/65490> for more information
    = help: add `#![feature(bindings_after_at)]` to the crate attributes to enable