[RFA/dwarf2] Small problem scanning line table for included files

[Jim Blandy]

This is getting a bit hairy.  It would be simpler to simply delete the
"!decode_for_pst_p && " towards the top of dwarf2_decode_lines, and
always create a psymtab for the first file.  This could conceivably
result in creating psymtabs for source files that have no line number
entries in them, but that's probably not going to be a big deal.

(To be picky, we can make that mistake now anyway: consider what would
happen if the line number program contained two consecutive
DW_LNS_set_file instructions: the first file gets a psymtab, even
though it's never used.)

To get everything exactly right, we would need to delay setting a file
table entry's included_p flag until just before we actually call
record_line.

What do you think?