cmd/compile/internal/gc: recursive-descent parser

This is a translation of the yacc-based parser with adjustements
to make the grammar work for a recursive-descent parser followed
by cleanups and simplifications.

The yacc actions were mostly literally copied for correctness
with better temporary names.

A few of the syntax tests were adjusted for slightly different
error messages (it is very difficult to match the yacc-based
error messages in all cases, and sometimes the new parser could
produce better errors).

The new parser is enabled by default.
To switch back to the yacc-based parser, set -oldparser.
To hardwire the switch back, uncomment "oldparser = 1" in lex.go.

- passes all.bash
- ~18% reduced parse time per file on average for make.bash
- ~3% reduced compile time for building cmd/compile

Change-Id: Icb5651bb9d8b9f66261762d2c94a03793050d4ce
Reviewed-on: https://go-review.googlesource.com/16665
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/compile/internal/gc/subr.go

@@ -34,7 +34,7 @@ func errorexit() {
 }
 
 func parserline() int {
-	if parsing && theparser.Lookahead() > 0 {
+	if oldparser != 0 && parsing && theparser.Lookahead() > 0 {
 		// parser has one symbol lookahead
 		return int(prevlineno)
 	}