go/types: simplify error messages for untyped value assignability

CL 242083 corrected an inaccurate error message related to the
assignability of untyped constant values. Previously the error message
was of the form "cannot convert ... to ...", which is misleading when
there is no explicit conversion in the syntax. The new error message
corrected this to "cannot use ... as ... in ...", but also appended an
inner error message that can be quite verbose. For example:

  cannot use "123" (untyped string constant) as int value in assignment:
  cannot convert "123" (untyped string constant) to int"

This might be more accurate, but is a regression in readability. Correct
this by only including the inner error message in the rare cases where
it is helpful: if the constant value overflows or is truncated.

For golang/go#22070

Change-Id: I8b8ee6ef713f64facc319894be09398b0b5ea500
Reviewed-on: https://go-review.googlesource.com/c/go/+/267717
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
Trust: Robert Griesemer <gri@golang.org>
Trust: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>

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