Unify read_initial_length implementations

Unify read_initial_length implementations

There are two implementations of read_initial_length in gdb.  This
merges them and moves the resulting function to leb.c.

2020-02-08  Tom Tromey  <tom@tromey.com>

	* dwarf2/read.c (read_initial_length): Move to leb.c.
	* dwarf2/leb.h (read_initial_length): Declare.
	* dwarf2/leb.c (read_initial_length): Move from read.c.  Add
	handle_nonstd parameter.
	* dwarf2/frame.c (read_initial_length): Remove.
	(decode_frame_entry_1): Update.

Change-Id: I34d37bad0f8a584bfa781432cba25e05e1bd5750