BUG #15516: Identifier not quoted with uppercase letter (spanish Ñ) doesn't get transformed to lowercase (ñ)

The following bug has been logged on the website:

Bug reference:      15516
Logged by:          Paco Calvo
Email address:      fcaa@tragsa.es
PostgreSQL version: 9.5.15
Operating system:   MS Windows Server 2012 R2
Description:

Hi list,

When using an identifier not quoted which includes an uppercase letter
(Spanish Ñ, Unicode U+00D1), it doesn't get transformed to lowercase (ñ,
Unicode U+00F1) as expected. For instance, using a database with UTF-8
encoding and both LC_COLLATE and LC_CTYPE as 'Spanish_Spain.1252', the
following sentence which uses 2 identifiers with a Ñ uppercase letter

  CREATE TABLE AÑO (CaÑo integer)

creates the table maintaining both Ñ letters as uppercase and forcing to
quote both identifiers as

  CREATE TABLE "aÑo" ("caÑo" integer)

instead of all lowercase and no quoting as expected

  CREATE TABLE año (caño integer)

This quirk in lowercasing identifiers forces to use a different, quoted
identifier or manually change afterwards every identifier.

NB: Reproduced also with PostgreSQL 9.6.9 on MS Windows Server 2012 R2 and
PostgreSQL 11.1 on MS Windows Server 2008 R2. Waiting to test it under Linux
and MacOS.