[PR gdb/19893] Fix handling of synthetic C++ references

[PR gdb/19893] Fix handling of synthetic C++ references

https://sourceware.org/bugzilla/show_bug.cgi?id=19893

I've traced the main source of the problem to pieced_value_funcs.coerce_ref not being
implemented. Since gdb always assumes references are implemented as pointers, this
causes it to think that it's dealing with a NULL pointer, thus breaking any operations
involving synthetic references.

What I did here was implementing pieced_value_funcs.coerce_ref using some of the synthetic
pointer handling code from indirect_pieced_value, as Pedro suggested. I also made a few
adjustments to the reference printing code so that it correctly shows either the address
of the referenced value or (if it's non-addressable) the "<synthetic pointer>" string.

I also wrote some unit tests based on Dwarf::assemble; these took a while to make
because in most cases I needed a synthetic reference to a physical variable. Additionally,
I started working on a unit test for classes that have a vtable, but ran into a few issues
so that'll probably go in a future patch. One thing that should definitely be fixed is that
proc function_range (called for MACRO_AT_func) will always try to compile/link using gcc
with the default options instead of g++, thus breaking C++ compilations that require e.g. libstdc++.

gdb/ChangeLog:

	* dwarf2loc.c (coerce_pieced_ref, indirect_synthetic_pointer,
	fetch_const_value_from_synthetic_pointer): New functions.
	(indirect_pieced_value): Move lower half to indirect_synthetic_pointer.
	(pieced_value_funcs): Implement coerce_ref.
	* valops.c (value_addr): Call coerce_ref for synthetic references.
	* valprint.c (valprint_check_validity): Return true for synthetic
	references.  Also, don't show "<synthetic pointer>" if they reference
	addressable values.
	(generic_val_print_ref): Handle synthetic references.  Also move some
	code to print_ref_address.
	(print_ref_address, get_value_addr_contents): New functions.

gdb/testsuite/ChangeLog:

	* gdb.dwarf2/implref.exp: Rename to...
	* gdb.dwarf2/implref-const.exp: ...this.  Also add more test statements.
	* gdb.dwarf2/implref-array.c: New file.
	* gdb.dwarf2/implref-array.exp: Likewise.
	* gdb.dwarf2/implref-global.c: Likewise.
	* gdb.dwarf2/implref-global.exp: Likewise.
	* gdb.dwarf2/implref-struct.c: Likewise.
	* gdb.dwarf2/implref-struct.exp: Likewise.