cmd/compile/internal/syntax: parser to accept ~x as unary expression

Accept ~x as ordinary unary expression in the parser but recognize
such expressions as invalid in the type checker.

This change opens the door to recognizing complex type constraint
literals such as `*E|~int` in `[P *E|~int]` and parse them correctly
instead of reporting a parse error because `P*E|~int` syntactically
looks like an incorrect array length expression (binary expression
where the RHS of | is an invalid unary expression ~int).

As a result, the parser is more forgiving with expressions but the
type checker will reject invalid uses as before.

We could pass extra information into the binary/unary expression
parse functions to prevent the use of ~ in invalid situations but
it doesn't seem worth the trouble. In fact it may be advantageous
to allow a more liberal expression syntax especially in the presence
of errors (better parser synchronization after an error).

Preparation for fixing #49482.

Change-Id: I119e8bd9445dfa6460fcd7e0658e3554a34b2769
Reviewed-on: https://go-review.googlesource.com/c/go/+/402255
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>

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

@@ -827,7 +827,7 @@ func (p *parser) unaryExpr() Expr {
 	switch p.tok {
 	case _Operator, _Star:
 		switch p.op {
-		case Mul, Add, Sub, Not, Xor:
+		case Mul, Add, Sub, Not, Xor, Tilde:
 			x := new(Operation)
 			x.pos = p.pos()
 			x.Op = p.op