Introduce accessors for psymtab high and low fields

Introduce accessors for psymtab high and low fields

This introduces accessors for the partial symbol table textlow and
texthigh fields.  This lets us later arrange to relocate these values
at their point of use.

I did this conversion by renaming the fields.  I didn't rename the
fields back afterward, thinking that on the off chance that someone
has a patch touching this area, then a merge would helpfully break
their compile.

I looked at making the fields private, but this interferes with the
memset in allocate_psymtab, and I didn't want to chase this down.
This conversion can be done later if need be.

gdb/ChangeLog
2018-07-26  Tom Tromey  <tom@tromey.com>

	* dbxread.c (read_dbx_symtab, end_psymtab, read_ofile_symtab):
	Update.
	* dwarf2read.c (dwarf2_create_include_psymtab): Don't initialize
	textlow and texthigh fields.
	(process_psymtab_comp_unit_reader, dwarf2_build_include_psymtabs):
	Update.
	* mdebugread.c (parse_lines, parse_partial_symbols)
	(psymtab_to_symtab_1): Update.
	* psympriv.h (struct partial_symtab) <m_text_low, m_text_high>:
	Rename fields.  Update comment.  Now private.
	<text_low, text_high, set_text_low, set_text_high>: New methods.
	* psymtab.c (find_pc_sect_psymtab_closer, find_pc_sect_psymtab)
	(find_pc_sect_psymbol, relocate_psymtabs, dump_psymtab)
	(start_psymtab_common, maintenance_info_psymtabs)
	(maintenance_check_psymtabs): Update.
	* xcoffread.c (xcoff_end_psymtab): Don't initialize textlow and
	texthigh fields.
	(scan_xcoff_symtab): Update.