[gdb] Skip imports of c++ CUs

The DWARF standard appendix E.1 describes techniques that can be used for
compression and deduplication: DIEs can be factored out into a new compilation
unit, and referenced using DW_FORM_ref_addr.

Such a new compilation unit can either use a DW_TAG_compile_unit or
DW_TAG_partial_unit.  If a DW_TAG_compile_unit is used, its contents is
evaluated by consumers as though it were an ordinary compilation unit.  If a
DW_TAG_partial_unit is used, it's only considered by consumers in the context
of a DW_TAG_imported_unit.

An example of when DW_TAG_partial_unit is required is when the factored out
DIEs are not top-level, f.i. because they were children of a namespace.  In
such a case the corresponding DW_TAG_imported_unit will occur as child of the
namespace.

In the case of factoring out DIEs from c++ compilation units, we can factor
out into a new DW_TAG_compile_unit, and no DW_TAG_imported_unit is required.

This begs the question how to interpret a top-level DW_TAG_imported_unit of a
c++ DW_TAG_compile_unit compilation unit.  The semantics of
DW_TAG_imported_unit describe that the imported unit logically appears at the
point of the DW_TAG_imported_unit entry.  But it's not clear what the effect
should be in this case, since all the imported DIEs are already globally
visible anyway, due to the use of DW_TAG_compile_unit.

So, skip top-level imports of c++ DW_TAG_compile_unit compilation units in
process_imported_unit_die.

Using the cc1 binary from PR23710 comment 1 and setting a breakpoint on do_rpo_vn:
...
$ gdb \
    -batch \
    -iex "maint set dwarf max-cache-age 316" \
    -iex "set language c++" \
    -ex "b do_rpo_vn" \
    cc1
...
we get a 8.1% reduction in execution time, due to reducing the number of
partial symtabs expanded into full symtabs from 212 to 175.

Build and reg-tested on x86_64-linux.

gdb/ChangeLog:

2020-03-17  Tom de Vries  <tdevries@suse.de>

	PR gdb/23710
	* dwarf2/read.h (struct dwarf2_per_cu_data): Add unit_type and lang
	fields.
	* dwarf2/read.c (process_psymtab_comp_unit): Initialize unit_type and lang
	fields.
	(process_imported_unit_die): Skip import of c++ CUs.