Simplify the psymbol hash function

Simplify the psymbol hash function

This patch simplifies the psymbol_hash function, by changing it not to
examine the contents of the symbol's name.  This change just mirrors
what psymbol_compare already does -- it is checking for name equality,
which is ok because symbol names are interned in symbol_set_names.

This change speeds up psymbol reading.  "gdb -nx -batch gdb"
previously took ~1.8 seconds on my machine, and with this patch it now
takes ~1.7 seconds.

gdb/ChangeLog
2017-11-09  Tom Tromey  <tom@tromey.com>

	* psymtab.c (psymbol_hash): Do not hash string contents.
	(psymbol_compare): Add comment.