[Ada] Fix bug in casing on composite values with -gnatX

[Pierre-Marie de Rodat]

Fix a bug (which shows up as an assertion failure in gigi) having to
do with the tree generated by the FE for a record-valued case-statement
choice that has a subcomponent of an enumeration type.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* exp_ch5.adb
	(Expand_N_Case_Statement.Expand_General_Case_Statement.Pattern_Match):
	When generating an equality test for a statically known discrete
	value, only generate the numeric value if the discrete type is
	not an enumeration type.  If it is an enumeration type, then
	call Get_Enum_Lit_From_Pos instead.
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