[Ada] Clean up of index usage in string literal subtypes

[Arnaud Charlet <charlet at adacore dot com>]

This patch adds code to retrieve the index type of a string literal. Since
string literals do not use attribute First_Index, the proper index type is
obtained from their low bound. No changes in compiler behavior.

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

2012-04-26  Hristian Kirtchev  <kirtchev@adacore.com>

	* einfo.adb, einfo.ads: Remove synthesized attribute
	Proper_First_Index along with its associations in various nodes.
	(Proper_First_Index): Removed.
	* sem_ch4.adb (Analyze_Slice): Alphabetize constants. Add new
	local variable Index_Type. The index type of a string literal
	subtype is that of the stored low bound.
	* sem_eval (Get_Static_Length): Remove the use of Proper_First_Index.
	* sem_res.adb (Resolve_Slice): Alphabetize constants. Add
	new local variable Index_Type. The index type of a
	string literal subtype is that of the stored low bound.
	(Set_String_Literal_Subtype): Code reformatting.

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