According to the language spec, a struct field name should
be an identifier.

  identifier = letter { letter | unicode_digit } .
  letter = unicode_letter | "_" .

Implements a function 'isValidFieldName(fieldName string) bool'.
To check if the field name is a valid identifier or not.
It will panic if the field name is invalid.

It uses the non-exported function implementation 'isLetter'
from the package 'scanner', used to parse an identifier.

Fixes #20600.

Change-Id: I1db7db1ad88cab5dbea6565be15cc7461cc56c44
Reviewed-on: https://go-review.googlesource.com/45590Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>