go/types, types2: allow choice between reg. expression or not in ERROR test comments

[griesemer]

go/types and types2 (and the syntax package) rely on ERROR comments in test cases. Such a comment accepts a regular expression pattern that is then matched against the expected error:

// ERROR regexp pattern

Often we just want to see if the error string appears verbatim (as a substring) in the error output, we don't need reg. expression matching. If we could choose a non-regexp string in those cases, we wouldn't need to escape various characters, which would make these ERROR comments more readable.

Example:

// ERROR \*T2 does not implement I1 \(wrong type for method foo\)\n\t\thave foo\(x int\)\n\t\twant foo\(\)

would become

// ERROR *T2 does not implement I1 (wrong type for method foo)\n\t\thave foo(x int)\n\t\twant foo()

The non-regexp pattern might be the more common case. Maybe we could have ERROR and ERRORx (the latter is for regexp. matching).

cc: @findleyr