Make top-level rustc_parse functions fallible.

Currently we have an awkward mix of fallible and infallible functions:
```
       new_parser_from_source_str
 maybe_new_parser_from_source_str
       new_parser_from_file
(maybe_new_parser_from_file)        // missing
      (new_parser_from_source_file) // missing
 maybe_new_parser_from_source_file
       source_str_to_stream
 maybe_source_file_to_stream
```
We could add the two missing functions, but instead this commit removes
of all the infallible ones and renames the fallible ones leaving us with
these which are all fallible:
```
new_parser_from_source_str
new_parser_from_file
new_parser_from_source_file
source_str_to_stream
source_file_to_stream
```
This requires making `unwrap_or_emit_fatal` public so callers of
formerly infallible functions can still work.

This does make some of the call sites slightly more verbose, but I think
it's worth it for the simpler API. Also, there are two `catch_unwind`
calls and one `catch_fatal_errors` call in this diff that become
removable thanks this change. (I will do that in a follow-up PR.)

Author: nnethercote

src/parse/parser.rs

@@ -4,7 +4,8 @@ use std::path::{Path, PathBuf};
 use rustc_ast::token::TokenKind;
 use rustc_ast::{ast, attr, ptr};
 use rustc_errors::Diag;
-use rustc_parse::{new_parser_from_file, parser::Parser as RawParser};
+use rustc_parse::parser::Parser as RawParser;
+use rustc_parse::{new_parser_from_file, unwrap_or_emit_fatal};
 use rustc_span::{sym, Span};
 use thin_vec::ThinVec;
 
@@ -68,10 +69,10 @@ impl<'a> ParserBuilder<'a> {
     ) -> Result<rustc_parse::parser::Parser<'a>, Option<Vec<Diag<'a>>>> {
         match input {
             Input::File(ref file) => catch_unwind(AssertUnwindSafe(move || {
-                new_parser_from_file(psess, file, None)
+                unwrap_or_emit_fatal(new_parser_from_file(psess, file, None))
             }))
             .map_err(|_| None),
-            Input::Text(text) => rustc_parse::maybe_new_parser_from_source_str(
+            Input::Text(text) => rustc_parse::new_parser_from_source_str(
                 psess,
                 rustc_span::FileName::Custom("stdin".to_owned()),
                 text,
@@ -111,7 +112,8 @@ impl<'a> Parser<'a> {
         span: Span,
     ) -> Result<(ast::AttrVec, ThinVec<ptr::P<ast::Item>>, Span), ParserError> {
         let result = catch_unwind(AssertUnwindSafe(|| {
-            let mut parser = new_parser_from_file(psess.inner(), path, Some(span));
+            let mut parser =
+                unwrap_or_emit_fatal(new_parser_from_file(psess.inner(), path, Some(span)));
             match parser.parse_mod(&TokenKind::Eof) {
                 Ok((a, i, spans)) => Some((a, i, spans.inner_span)),
                 Err(e) => {