Gate async fn trait bound modifier on async_trait_bounds

[compiler-errors]

This PR moves async Fn() trait bounds into a new feature gate: feature(async_trait_bounds). The general vibe is that we will most likely stabilize the feature(async_closure) without the async Fn() trait bound modifier, so we need to gate that separately.

We're trying to work on the general vision of async trait bound modifier general in: rust-lang/rfcs#3710, however that RFC still needs more time for consensus to converge, and we've decided that the value that users get from calling the bound async Fn() is not really worth blocking landing async closures in general.

This is blocked on #132611, since moving these tests to AsyncFn is annoying without a prelude import.

r? @ghost

[rust-log-analyzer]

The job x86_64-gnu-llvm-18 failed! Check out the build log: (web) (plain)

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 > importing cache manifest from ghcr.io/rust-lang/rust-ci-cache:c32c805632780b5c1de330e3f44561b336c2efe163bc0990acb392390157a8e1d9f855d75914a239aa40c49d77f4a837247d05d2f8d46f554b98e1f46712a3e3:
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##[endgroup]
Setting extra environment values for docker:  --env ENABLE_GCC_CODEGEN=1 --env GCC_EXEC_PREFIX=/usr/lib/gcc/
[CI_JOB_NAME=x86_64-gnu-llvm-18]
debug: `DISABLE_CI_RUSTC_IF_INCOMPATIBLE` configured.
---
sccache: Starting the server...
##[group]Configure the build
configure: processing command line
configure: 
configure: build.configure-args := ['--build=x86_64-unknown-linux-gnu', '--llvm-root=/usr/lib/llvm-18', '--enable-llvm-link-shared', '--set', 'rust.randomize-layout=true', '--set', 'rust.thin-lto-import-instr-limit=10', '--enable-verbose-configure', '--enable-sccache', '--disable-manage-submodules', '--enable-locked-deps', '--enable-cargo-native-static', '--set', 'rust.codegen-units-std=1', '--set', 'dist.compression-profile=balanced', '--dist-compression-formats=xz', '--set', 'rust.lld=false', '--disable-dist-src', '--release-channel=nightly', '--enable-debug-assertions', '--enable-overflow-checks', '--enable-llvm-assertions', '--set', 'rust.verify-llvm-ir', '--set', 'rust.codegen-backends=llvm,cranelift,gcc', '--set', 'llvm.static-libstdcpp', '--enable-new-symbol-mangling']
configure: target.x86_64-unknown-linux-gnu.llvm-config := /usr/lib/llvm-18/bin/llvm-config
configure: llvm.link-shared     := True
configure: rust.randomize-layout := True
configure: rust.thin-lto-import-instr-limit := 10
---
##[group]Testing stage2 compiletest suite=crashes mode=crashes (x86_64-unknown-linux-gnu)

running 238 tests
........................................................................................  88/238
..................2024-11-06T20:12:56.417723Z ERROR compiletest::runtest: fatal error, panic: "crashtest no longer crashes/triggers ICE, horray! Please give it a meaningful name, add a doc-comment to the start of the test explaining why it exists and move it to tests/ui or wherever you see fit. Adding 'Fixes #<issueNr>' to your PR description ensures that the corresponding ticket is auto-closed upon merge. If you want to see verbose output, set `COMPILETEST_VERBOSE_CRASHES=1`."
[crashes] tests/crashes/124020.rs ... F
..................................................................... 176/238
..............................................................


failures:

---- [crashes] tests/crashes/124020.rs stdout ----

error: crashtest no longer crashes/triggers ICE, horray! Please give it a meaningful name, add a doc-comment to the start of the test explaining why it exists and move it to tests/ui or wherever you see fit. Adding 'Fixes #<issueNr>' to your PR description ensures that the corresponding ticket is auto-closed upon merge. If you want to see verbose output, set `COMPILETEST_VERBOSE_CRASHES=1`.
thread '[crashes] tests/crashes/124020.rs' panicked at src/tools/compiletest/src/runtest/crashes.rs:17:18:
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


failures: