dwarf2out.c (prefix_of): New function.

dwarf2out.c (prefix_of): New function.

	* dwarf2out.c (prefix_of): New function.  Determine longest common
	prefix of the two arguments.  The units are whole path components.
	(output_file_names): When adding a new directory find the one entry
	with the longest common prefix already in the list.  Artificially
	generate entry if there is none for this prefix.
	Fix test to check whether the zeroth directory entry is used.
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From-SVN: r39253