go/parser: parser doesn't recognize valid type parameter list

[griesemer]

This is the go/parser version of issue #49482 so we can track the fixes separately as the go/parser changes are not a trivial port of the syntax parser changes.

[gopherbot]

Change https://go.dev/cl/403696 mentions this issue: go/parser: parser to accept ~x as unary expression

[gopherbot]

Change https://go.dev/cl/403937 mentions this issue: go/parser: accept all valid type parameter lists

[gopherbot]

Change https://go.dev/cl/404194 mentions this issue: go/printer: fix printing for possibly ambiguous type constraints