[Ada] Replace chopped string copy with renaming

[Pierre-Marie de Rodat]

Avoid local string copy with renaming, which both make the code shorter
to read and should be marginally faster to execute. Code cleanup only
related to loading of compilation units; semantics is unaffected.

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

gcc/ada/

	* par.adb (Par): A local Name variable is now a renaming of a
	constant slice.
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