Raw fluent diagnostic structs

[davidtwco]

Alternative implementation of #117867. Need to decide which we would like more.

[rustbot]

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[davidtwco]

#120576 makes this a bit simpler for subdiagnostics, as those would take a DiagnosticBuilder after that PR.

[davidtwco]

This doesn't need to be reviewed until we decide what we want to do with these structs.

[rust-log-analyzer]

The job x86_64-gnu-llvm-16 failed! Check out the build log: (web) (plain)

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

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GITHUB_EVENT_NAME=pull_request
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GITHUB_GRAPHQL_URL=https://api.github.com/graphql
GITHUB_HEAD_REF=raw-fluent-diagnostics
GITHUB_JOB=pr
GITHUB_PATH=/home/runner/work/_temp/_runner_file_commands/add_path_1fba62a6-3fe0-4882-82ac-33b50ecbf586
GITHUB_REF=refs/pull/121334/merge
GITHUB_REF_NAME=121334/merge
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---
#12 writing image sha256:898b006d88f32f734530b47a86ec4b69fab1099784ec71c50f2797978e80f265 done
#12 naming to docker.io/library/rust-ci done
#12 DONE 10.2s
##[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-16]
##[group]Clock drift check
  local time: Tue Feb 20 10:12:09 UTC 2024
  network time: Tue, 20 Feb 2024 10:12:10 GMT
  network time: Tue, 20 Feb 2024 10:12:10 GMT
##[endgroup]
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-16', '--enable-llvm-link-shared', '--set', 'rust.thin-lto-import-instr-limit=10', '--set', 'change-id=99999999', '--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', 'build.optimized-compiler-builtins', '--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-16/bin/llvm-config
configure: llvm.link-shared     := True
configure: rust.thin-lto-import-instr-limit := 10
configure: change-id            := 99999999

[bors]

☔ The latest upstream changes (presumably #120576) made this pull request unmergeable. Please resolve the merge conflicts.