cmd/compile: handle pragmas immediately with -newparser=1

Instead of saving all pragmas and processing them after parsing is
finished, process them immediately during scanning like the current
lexer does.

This is a bit unfortunate because it means we can't use
syntax.ParseFile to concurrently parse files yet, but it fixes how we
report syntax errors in the presence of //line pragmas.

While here, add a bunch more gcCompat entries to syntax/parser.go to
get "go build -toolexec='toolstash -cmp' std cmd" passing. There are
still a few remaining cases only triggered building unit tests, but
this seems like a nice checkpoint.

Change-Id: Iaf3bbcf2849857a460496f31eea228e0c585ce13
Reviewed-on: https://go-review.googlesource.com/28226
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/cmd/compile/internal/syntax/scanner_test.go

@@ -22,7 +22,7 @@ func TestScanner(t *testing.T) {
 	defer src.Close()
 
 	var s scanner
-	s.init(src, nil)
+	s.init(src, nil, nil)
 	for {
 		s.next()
 		if s.tok == _EOF {
@@ -51,7 +51,7 @@ func TestTokens(t *testing.T) {
 
 	// scan source
 	var got scanner
-	got.init(&bytesReader{buf}, nil)
+	got.init(&bytesReader{buf}, nil, nil)
 	got.next()
 	for i, want := range sampleTokens {
 		nlsemi := false
@@ -338,7 +338,7 @@ func TestScanErrors(t *testing.T) {
 			} else if nerrors > 1 {
 				t.Errorf("%q: got unexpected %q at pos = %d, line = %d", test.src, msg, pos, line)
 			}
-		})
+		}, nil)
 
 		for {
 			s.next()