Consolidate partial symtab dependency reading

Consolidate partial symtab dependency reading

Most of the symbol readers have code to iterate over a partial symtabs
dependencies, expanding each one and optionally printing a message.
Now that the "second-stage" psymtab expansion is available as a
method, these implementations can all be merged.

This patch also changes a couple more warnings into assertions.

gdb/ChangeLog
2020-01-26  Tom Tromey  <tom@tromey.com>

	* xcoffread.c (xcoff_psymtab_to_symtab_1): Call
	read_dependencies.  Add assert.
	* psymtab.c (partial_symtab::read_dependencies): New method.
	* psympriv.h (struct partial_symtab) <read_dependencies>: New
	method.
	* mdebugread.c (psymtab_to_symtab_1): Call read_dependencies.
	* dwarf2read.c (dwarf2_psymtab::expand_psymtab): Call
	read_dependencies.
	* dbxread.c (dbx_psymtab_to_symtab_1): Call read_dependencies.
	Add assert.

Change-Id: I8151e05677794e90223edc1a4cb70f7f69137d46