Isolate STABS readers' use of the `textlow' and `texthigh' fields

Isolate STABS readers' use of the `textlow' and `texthigh' fields
of `struct partial_symtab' to only a few locations.  This change
is not supposed to affect the way the values are computed, only
where they live.

* dbxread.c (struct symloc): Add `textlow' and `texthigh' fields
to the reader-specific structure.
* mdebugread.c (struct symloc): Same.
* dbxread.c (TEXTLOW, TEXTHIGH): New accessor macros.
* mdebugread.c (TEXTLOW, TEXTHIGH): Same.
* dbxread.c (dbx_symfile_read): After we've built all our partial
symbol tables, set each partial symtab's `textlow' and `texthigh'
fields from our reader-specific structure.
* mdebugread.c (mdebug_build_psymtabs): Same.
* dbxread.c (start_psymtab): Initialize the reader-specific
structure's `textlow' and `texthigh' from the new psymtab's.
* mdebugread.c (parse_partial_symbols, new_psymtab): Same.
* dbxread.c (read_dbx_symtab, end_psymtab, read_ofile_symtab): Use
the reader-specific `textlow' and `texthigh', not the generic
psymtab fields.
* mdebugread.c (parse_lines, parse_partial_symbols,
psymtab_to_symtab_1): Same.
* partial-stab.h: Same.