External validation constraint must be defined within a numeric type

[alexander-yevsyukov]

We should not allow min/max value of an int32 field defined as "3.14". Such a definition is a programming error, and should be reported as such on compile-time.

[armiol]

@dmdashenkov please have a look. I am not sure we support "compile-time" checks for the external validation constraints.

[dmdashenkov]

This is handled in all the cases except two:

• validation code generation is disabled;
• external constraints.