[Ada] Remove repeated routines for printing AST in Mixed_Case

[Pierre-Marie de Rodat]

Code cleanup; behaviour is unaffected.

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

gcc/ada/

	* osint.adb (To_Lower): Clarify that only To_Lower function
	causes bootstrap issues; fix style.
	* treepr.adb (Print_Str_Mixed_Case): Reuse existing case
	conversion routine.
	(To_Mixed): Rename from Capitalize; reuse System.Case_Util
	procedure and explain the bootstrap issue.
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