fix: incorrect error message for string literal suffixes

[notJoon]

Description

Fixes #144161

When a raw string literal has an invalid suffix (like r#"test"#suffix), the compiler was not reporting the appropriate error message "suffixes on string literals are invalid". This was particularly problematic when the suffix was immediately followed by another token, as in r#" \\ "#r"\\ ".

Changes

Modified parse_expr_lit in compiler/rustc_parse/src/parser/expr.rs to validate literals using LitKind::from_token_lit and report errors appropriately.

By doing so,

• Single point of error reporting prevents duplicates
• Preserves the original token for attribute parsing

When modifying the lexer directly to detect invalid suffixes, the two problems mentioned above additionally occurred.

Also, modified it to consume the next token if it's a string literal after reporting a suffix error. This prevents the parser from generating the cascading error.

[rust-log-analyzer]

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tidy check
Checking tidy rustdoc_json...
No error code explanation was removed!
tidy: Skipping binary file check, read-only filesystem
##[error]tidy error: /checkout/tests/ui/parser/raw-string-suffix-invalid.rs:2: line longer than 100 chars
some tidy checks failed
Command `/checkout/obj/build/aarch64-unknown-linux-gnu/stage1-tools-bin/rust-tidy /checkout /checkout/obj/build/aarch64-unknown-linux-gnu/stage0/bin/cargo /checkout/obj/build 4 npm` failed with exit code 1
Created at: src/bootstrap/src/core/build_steps/tool.rs:1583:23
Executed at: src/bootstrap/src/core/build_steps/test.rs:1225:29

Command has failed. Rerun with -v to see more details.
Bootstrap failed while executing `--stage 2 test --skip tests --skip coverage-map --skip coverage-run --skip library --skip tidyselftest`
Build completed unsuccessfully in 0:00:52
  local time: Tue Aug 19 15:20:11 UTC 2025
  network time: Tue, 19 Aug 2025 15:20:11 GMT
##[error]Process completed with exit code 1.