spec: non-integral constant can be converted to int

[dvyukov]

The program is:

package a
const a int = 1-1/1e100
const b int = 1-1/1e300

gotype rejects both assignments.
gc rejects only the first one with "constant 1 truncated to integer", but accepts the second one.

The spec says:

This rounding may cause a floating-point constant expression to be invalid in an integer context, even if it would be integral when calculated using infinite precision.

http://golang.org/ref/spec#Constant_expressions
But it does not say the opposite -- FP const may be valid in int context, even if it is not integral when calculated using infinite precision. I think it should say so.

The second point is that "constant 1 truncated to integer" error does not make sense.

[griesemer]

https://go-review.googlesource.com/11369 addresses the spec issue.

Split out #11371 as a separate cmd/compile error.