cmd/compile/internal/syntax: fix printing of ambiguous constraint literals
Without this change, the type parameter list "[P T | T]" is printed
as "[P T | T,]" in an attempt to avoid an ambiguity. But the type
parameter P cannot syntactically combine with the constraint T | T
and make a new valid expression.
This change introduces a specific combinesWithName predicate that
reports whether a constraint expression can combine with a type
parameter name to form a new valid (value) expression.
Use combinesWithName to accurately determine when a comma is needed.
For #49482.
Change-Id: Id1d17a18f0c9af04495da7b0453e83798f32b04a
Reviewed-on: https://go-review.googlesource.com/c/go/+/404397 Reviewed-by: Ian Lance Taylor <iant@google.com> Reviewed-by: Robert Findley <rfindley@google.com>