gdb: Remove a use of VEC from dwarf2read.{c,h}

gdb: Remove a use of VEC from dwarf2read.{c,h}

Removes a use of VEC from dwarf2read.{c,h} and replaces it with
std::vector.  As far as possible this is a like for like replacement
with minimal refactoring.

There should be no user visible changes after this commit.

gdb/ChangeLog:

	* dwarf2read.c (struct type_unit_group) <tus>: Convert to
	std::vector.
	(build_type_psymtabs_reader): Update for std::vector.
	(build_type_psymtab_dependencies): Likewise.
	* dwarf2read.h: Remove use of DEF_VEC_P.
	(typedef sig_type_ptr): Delete.