Better diagnostic for using struct fields in consts

[JulianKnodt]

Addresses #92776, by printing out a help message if an error was raised when a . was encountered in a type.
cc @estebank since it's diagnostics related

[rust-highfive]

r? @nagisa

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[rust-log-analyzer]

The job mingw-check failed! Check out the build log: (web) (plain)

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

configure: rust.debug-assertions := True
configure: rust.overflow-checks := True
configure: llvm.assertions      := True
configure: dist.missing-tools   := True
configure: build.configure-args := ['--enable-sccache', '--disable-manage-submodu ...
configure: writing `config.toml` in current directory
configure: 
configure: run `python /checkout/x.py --help`
configure: 
---
    Finished release [optimized] target(s) in 9.54s
tidy check
tidy: Skipping binary file check, read-only filesystem
Checking which error codes lack tests...
tidy error: following path contains more than 2310 entries, you should move the test to some relevant subdirectory (current: 2312): /checkout/src/test/ui/issues
* highest error code: E0786
Found 502 error codes
Found 0 error codes with no tests
Done!
Done!
* 361 features
some tidy checks failed


command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-tools-bin/rust-tidy" "/checkout" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "/checkout/obj/build" "16"


Build completed unsuccessfully in 0:00:13

[compiler-errors]

Hahah, I also put up #92884 to address the same issue. My bad for not claiming it when I got interested in it an hour ago. :^)

Feel free to steal my implementation from that PR. Just let me know if I should close my PR out in favor for yours, since you got here first.

[JulianKnodt]

@compiler-errors No I don't mind, feel free to take the content from this PR if you'd like, mine fails CI anyway and it takes me a few hours to fix things on my end so go ahead!