[Ada] Style check for mixed-case identifiers

[Ada] Style check for mixed-case identifiers

This patch implements a new switch, -gnatyD, enables a style check that
requires defining identifiers to be in mixed case.

2019-07-03  Bob Duff  <duff@adacore.com>

gcc/ada/

	* par-ch3.adb (P_Defining_Identifier): Call
	Check_Defining_Identifier_Casing.
	* style.ads, styleg.ads, styleg.adb
	(Check_Defining_Identifier_Casing): New procedure to check for
	mixed-case defining identifiers.
	* stylesw.ads, stylesw.adb (Style_Check_Mixed_Case_Decls): New
	flag for checking for mixed-case defining identifiers.
	* doc/gnat_ugn/building_executable_programs_with_gnat.rst:
	Document new feature.
	* gnat_ugn.texi: Regenerate.

From-SVN: r272972