Rollup merge of #125815 - nnethercote:rustc_parse-top-level-cleanups, r=spastorino

`rustc_parse` top-level cleanups

A bunch of improvements in and around `compiler/rustc_parse/src/lib.rs`. Many of the changes streamline the API in that file from this (12 functions and one macro):
```
    name                              args                  return type
    ----                              ----                  -----------
    panictry_buffer!                  Result<T, Vec<Diag>>  T

pub parse_crate_from_file             path                  PResult<Crate>
pub parse_crate_attrs_from_file       path                  PResult<AttrVec>
pub parse_crate_from_source_str       name,src              PResult<Crate>
pub parse_crate_attrs_from_source_str name,src              PResult<AttrVec>

pub new_parser_from_source_str        name,src              Parser
pub maybe_new_parser_from_source_str  name,src              Result<Parser, Vec<Diag>>
pub new_parser_from_file              path,error_sp         Parser
    maybe_source_file_to_parser       srcfile               Result<Parser, Vec<Diag>>

pub parse_stream_from_source_str      name,src,override_sp  TokenStream
pub source_file_to_stream             srcfile,override_sp   TokenStream
    maybe_file_to_stream              srcfile,override_sp   Result<TokenStream, Vec<Diag>>

pub stream_to_parser                  stream,subparser_name Parser
```
to this:
```
    name                              args                  return type
    ----                              ----                  -----------
    unwrap_or_emit_fatal              Result<T, Vec<Diag>>  T

pub new_parser_from_source_str        name,src              Result<Parser, Vec<Diag>>
pub new_parser_from_file              path,error_sp         Result<Parser, Vec<Diag>>
    new_parser_from_source_file       srcfile               Result<Parser, Vec<Diag>>

pub source_str_to_stream              name,src,override_sp  Result<TokenStream, Vec<Diag>>
    source_file_to_stream             srcfile,override_sp   Result<TokenStream, Vec<Diag>>
```
I found the old API quite confusing, with lots of similar-sounding function names and no clear structure. I think the new API is much better.

r? `@spastorino`

Author: workingjubilee

clippy_lints/src/doc/needless_doctest_main.rs

@@ -8,7 +8,7 @@ use rustc_data_structures::sync::Lrc;
 use rustc_errors::emitter::HumanEmitter;
 use rustc_errors::{Diag, DiagCtxt};
 use rustc_lint::LateContext;
-use rustc_parse::maybe_new_parser_from_source_str;
+use rustc_parse::new_parser_from_source_str;
 use rustc_parse::parser::ForceCollect;
 use rustc_session::parse::ParseSess;
 use rustc_span::edition::Edition;
@@ -50,7 +50,7 @@ pub fn check(
                 let sm = Lrc::new(SourceMap::new(FilePathMapping::empty()));
                 let psess = ParseSess::with_dcx(dcx, sm);
 
-                let mut parser = match maybe_new_parser_from_source_str(&psess, filename, code) {
+                let mut parser = match new_parser_from_source_str(&psess, filename, code) {
                     Ok(p) => p,
                     Err(errs) => {
                         errs.into_iter().for_each(Diag::cancel);