[Ada] exp_dbug fixes

[Arnaud Charlet <charlet at adacore dot com>]

Tested on i686-linux, committed on trunk

Modify Get_Encoded_Name function to continue providing its
functionality when the backend is generating code.
Otherwise any serious error reported by the backend calling the frontend
routine Error_Msg changes the Compilation_Mode to Check_Semantics, disables the
functionality of this routine and causes the generation of
spureous additional errors.

Update documentation in exp_dbug.ads

2007-06-06  Javier Miranda  <miranda@adacore.com>
	    Nicolas Setton  <setton@adacore.com>

	* exp_dbug.adb (Get_Encoded_Name): Modified to continue providing its
	functionality when the backend is generating code.
	Otherwise any serious error
	reported by the backend calling the frontend routine Error_Msg
	changes the Compilation_Mode to Check_Semantics, disables the
	functionality of this routine and causes the generation of
	spureous additional errors.

	* exp_dbug.ads (Pointers to Unconstrained Arrays): Document the
	debugging information now generated by the compiler for fat-pointer
	types.
	Document the contents of DW_AT_producer in the GNAT Vendor extensions to
	DWARF2/3.
	Document GNAT Vendor extensions to DWARF 2/3 and the "-gdwarf+" switch.

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