gc: allow tags on parameters in export section of object files.

This is in preparation of escape analysis; function parameters
can now be tagged with interesting bits by the compiler by
assigning to n->note.

tested by having typecheck put a fake tag on all parameters of
pointer type and compiling the tree.

R=rsc
CC=golang-dev
https://golang.org/cl/4524092

src/cmd/gc/go.y

@@ -1792,24 +1792,12 @@ hidden_opt_sym:
 	}
 
 hidden_dcl:
-	hidden_opt_sym hidden_type
+	hidden_opt_sym hidden_type hidden_tag
 	{
 		$$ = nod(ODCLFIELD, $1, typenod($2));
+		$$->val = $3;
 	}
-|	hidden_opt_sym LDDD
-	{
-		Type *t;
-
-		yyerror("invalid variadic function type in import - recompile import");
-		
-		t = typ(TARRAY);
-		t->bound = -1;
-		t->type = typ(TINTER);
-		$$ = nod(ODCLFIELD, $1, typenod(t));
-		$$->isddd = 1;
-	}
-
-|	hidden_opt_sym LDDD hidden_type
+|	hidden_opt_sym LDDD hidden_type hidden_tag
 	{
 		Type *t;
 		
@@ -1818,6 +1806,7 @@ hidden_dcl:
 		t->type = $3;
 		$$ = nod(ODCLFIELD, $1, typenod(t));
 		$$->isddd = 1;
+		$$->val = $4;
 	}
 
 hidden_structdcl:

src/pkg/go/types/gcimporter.go

@@ -403,7 +403,7 @@ func (p *gcParser) parseStructType() Type {
 }
 
 
-// Parameter = ( identifier | "?" ) [ "..." ] Type .
+// Parameter = ( identifier | "?" ) [ "..." ] Type [ ":" string_lit ] .
 //
 func (p *gcParser) parseParameter() (par *ast.Object, isVariadic bool) {
 	name := p.parseName()
@@ -415,6 +415,11 @@ func (p *gcParser) parseParameter() (par *ast.Object, isVariadic bool) {
 		isVariadic = true
 	}
 	ptyp := p.parseType()
+	// ignore argument tag
+	if p.tok == ':' {
+		p.next()
+		p.expect(scanner.String)
+	}
 	par = ast.NewObj(ast.Var, name)
 	par.Type = ptyp
 	return