When gcc dwarf2out generates the .debug_line table itself (for example
when generating one for a split DWARF .dwo) it uses natural sorting for
the directory table. Shorter directory paths come before longer directory
paths with the same prefix. This causes the files in the line table to
pick the shorter dir. Creating slightly ineffecient line tables because
the longer directory paths will never be used.

Fix this by changing file_info_cmp () to pick longer directory prefixes
before shorter ones. We still sort files (the compilation unit) without
any directory path before all entries with a directory path, so they
will still use dir entry 0 (the working directory).

A hello.c program would get the following dir and line table before:

Directory table:
 /opt/local/install/gcc/lib/gcc/x86_64-pc-linux-gnu/8.0.1/include
 /usr/include
 /usr/include/bits

File name table:
 Entry Dir   Time      Size      Name
 1     0     0         0         hello.c
 2     1     0         0         stddef.h
 3     2     0         0         bits/types.h
 4     2     0         0         libio.h
 5     2     0         0         stdio.h
 6     2     0         0         bits/sys_errlist.h

Note that the last directory table entry is never used.
After this patch it looks as follows:

Directory table:
 /opt/local/install/gcc/lib/gcc/x86_64-pc-linux-gnu/8.0.1/include
 /usr/include/bits
 /usr/include

File name table:
 Entry Dir   Time      Size      Name
 1     0     0         0         hello.c
 2     1     0         0         stddef.h
 3     2     0         0         types.h
 4     3     0         0         libio.h
 5     3     0         0         stdio.h
 6     2     0         0         sys_errlist.h

Which is similar to what gas would output.

gcc/ChangeLog:

	* dwarf2out.c (file_info_cmp): Sort longer dir prefixes before
	shorter ones.

From-SVN: r259655