gdb: Remove use of VEC from dwarf2read.c

gdb: Remove use of VEC from dwarf2read.c

This removes a use of VEC from GDB, from dwarf2read.c.  This removal
is not very clean, and would probably benefit from additional
refactoring in the future.

The problem here is that the VEC is contained within struct
dwarf2_per_cu_data, which is treated as POD in dwarf2read.c.  As such
it is actually a VEC pointer.  When converting this to a std::vector
in an ideal world we would not use a std::vector pointer, and use the
std::vector directly.  Sadly, to do that would require some rewriting
in dwarf2read.c - my concern would be introducing bugs during this
rewrite.

If we move to a std::vector pointer then we need to take care to
handle the case where the pointer is null.  The old VEC library would
handle null for us, making the VEC interface very clean.  With
std::vector we need to handle the null pointer case ourselves.

The achieve this then I've added a small number of function that wrap
up access to the std::vector, hopefully hiding the null pointer
management.

The final ugliness with this conversion is that, ideally, when
wrapping a data member behind an interface I would make the data
member private, however, treating the structure as POD once again
prevents this, so we are left with the data member being public, but
access (ideally) being through the published interface functions.

There should be no user visible changes after this commit.

gdb/ChangeLog:

	* gdb/dwarf2read.c (dwarf2_per_objfile::~dwarf2_per_objfile):
	Update for new std::vector based implementation.
	(process_psymtab_comp_unit_reader): Likewise.
	(scan_partial_symbols): Likewise.
	(recursively_compute_inclusions): Likewise.
	(compute_compunit_symtab_includes): Likewise.
	(process_imported_unit_die): Likewise.
	(queue_and_load_dwo_tu): Likewise.
	(follow_die_sig_1): Likewise.
	* gdb/dwarf2read.h: Remove DEF_VEC_P.
	(typedef dwarf2_per_cu_ptr): Remove.
	(struct dwarf2_per_cu_data) <imported_symtabs_empty>: New
	function.
	(struct dwarf2_per_cu_data) <imported_symtabs_push>: New function.
	(struct dwarf2_per_cu_data) <imported_symtabs_size>: New function.
	(struct dwarf2_per_cu_data) <imported_symtabs_free>: New function.
	(struct dwarf2_per_cu_data) <imported_symtabs>: Change to
	std::vector.

Change-Id: Id0f4bda977c9dd83b0ba3d7fb42f7e5e2b6869c8