Consolidate psymtab "Reading" messages

Consolidate psymtab "Reading" messages

Each symbol reader implemented its own "Reading..." messages, and most
of them double-checked that a previously-expanded psymtab could not be
re-read.

This patch consolidates the message-printing, and changes these checks
into asserts.

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

	* xcoffread.c (xcoff_read_symtab): Remove prints.  Add assert.
	* psymtab.c (psymtab_to_symtab): Print verbose "Reading"
	messages.
	* mdebugread.c (mdebug_read_symtab): Remove prints.
	* dwarf2read.c (dwarf2_psymtab::read_symtab): Remove prints.  Add
	assert.
	* dbxread.c (dbx_read_symtab): Remove prints.  Add assert.

Change-Id: I795be9710d42708299bb7b44972cffd27aec9413