cmd/compile: fix contrived line number errors

If a general comment contains multiple newline characters, we can't
simply unread one and then re-lex it via the general whitespace lexing
phase, because then we'll reset lineno to the line before the "*/"
marker, rather than keeping it where we found the "/*" marker.

Also, for processing imports, call importfile before advancing the
lexer with p.next(), so that lineno reflects the line where we found
the import path, and not the token afterwards.

Fixes #14520.

Change-Id: I785a2d83d632280113d4b757de0d57c88ba2caf4
Reviewed-on: https://go-review.googlesource.com/19934
Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -333,20 +333,22 @@ func (p *parser) importdcl() {
 	}
 
 	line := int32(parserline())
-	path := p.val
-	p.next()
 
-	importfile(&path, p.indent)
-	if importpkg == nil {
+	// We need to clear importpkg before calling p.next(),
+	// otherwise it will affect lexlineno.
+	// TODO(mdempsky): Fix this clumsy API.
+	importfile(&p.val, p.indent)
+	ipkg := importpkg
+	importpkg = nil
+
+	p.next()
+	if ipkg == nil {
 		if nerrors == 0 {
 			Fatalf("phase error in import")
 		}
 		return
 	}
 
-	ipkg := importpkg
-	importpkg = nil
-
 	ipkg.Direct = true
 
 	if my == nil {