Replace parse_[sth]_expr with parse_expr_[sth] function names

This resolves an inconsistency in naming style for functions
on the parser, between functions parsing specific kinds of items
and those for expressions, favoring the parse_item_[sth] style
used by functions for items. There are multiple advantages
of that style:

* functions of both categories are collected in the same place
  in the rustdoc output.
* it helps with autocompletion, as you can narrow down your
  search for a function to those about expressions.
* it mirrors rust's path syntax where less specific things
  come first, then it gets more specific, i.e.
  std::collections::hash_map::Entry

The disadvantage is that it doesn't "read like a sentence"
any more, but I think the advantages weigh more greatly.

This change was mostly application of this command:

sed -i -E 's/(fn |\.)parse_([[:alnum:]_]+)_expr/\1parse_expr_\2/' compiler/rustc_parse/src/parser/*.rs

Plus very minor fixes outside of rustc_parse, and an invocation
of x fmt.

compiler/rustc_builtin_macros/src/asm.rs

@@ -152,7 +152,7 @@ pub fn parse_asm_args<'a>(
                 ast::InlineAsmOperand::InOut { reg, expr, late: true }
             }
         } else if p.eat_keyword(kw::Const) {
-            let anon_const = p.parse_anon_const_expr()?;
+            let anon_const = p.parse_expr_anon_const()?;
             ast::InlineAsmOperand::Const { anon_const }
         } else if p.eat_keyword(sym::sym) {
             let expr = p.parse_expr()?;