Simplify setting of reading_partial_symbols

Simplify setting of reading_partial_symbols

This simplifies the setting and clearing of reading_partial_symbols,
by using scoped_restore in the function that reads partial symbols.

gdb/ChangeLog
2020-02-24  Tom Tromey  <tom@tromey.com>

	* dwarf2read.c (dwarf2_build_psymtabs_hard): Use
	make_scoped_restore.
	(dwarf2_psymtab::read_symtab): Don't clear
	reading_partial_symbols.