Add support for global rvalue initialization and constructors

This patch adds support for initialization of global variables
with rvalues and creating constructors for array, struct and
union types which can be used as rvalues.

Signed-off-by:
2021-12-14	Petter Tomner	<tomner@kth.se>

gcc/jit/
	* jit-common.h: New enum
	* jit-playback.c : Folding an setting intitial
	(global_new_decl) : Handle const global generation
	(new_global) : New flag
	(global_set_init_rvalue) : New
	(new_ctor) : New
	(new_global_initialized) : Flag
	(as_truth_value) : Fold
	(new_unary_op) : Fold
	(new_binary_op) : Fold
	(new_comparison) : Fold
	(new_array_access) : Fold
	(new_dereference) : Fold
	(get_address) : Fold
	* jit-playback.h :
	(global_set_init_rvalue) : New
	(new_ctor) : New
	* jit-recording.c :
	* jit-recording.h :
	(new_global_init_rvalue) : New
	(new_ctor) : New
	(ctor) : New, inherits rvalue
	(global_init_rvalue) : New, inherits memento
	(type::is_union) : New
	* libgccjit++.h : New entrypoints, see C-header
	* libgccjit.c : See .h
	* libgccjit.h : New entrypoints
	(gcc_jit_context_new_array_constructor) : New
	(gcc_jit_context_new_struct_constructor) : New
	(gcc_jit_context_new_union_constructor) : New
	(gcc_jit_global_set_initializer_rvalue) : New
	(LIBGCCJIT_HAVE_CTORS) : New feuture macro
	* libgccjit.map : New entrypoints added to ABI 19
	* docs/topics/expressions.rst : Updated docs

gcc/testsuite/
	* jit.dg/all-non-failing-tests.h: Added two tests
	* jit.dg/test-error-ctor-array-wrong-obj.c: New
	* jit.dg/test-error-ctor-struct-too-big.c: New
	* jit.dg/test-error-ctor-struct-wrong-field-obj.c: New
	* jit.dg/test-error-ctor-struct-wrong-type.c: New
	* jit.dg/test-error-ctor-struct-wrong-type2.c
	* jit.dg/test-error-ctor-union-wrong-field-name.c: New
	* jit.dg/test-error-global-already-init.c: New
	* jit.dg/test-error-global-common-section.c: New
	* jit.dg/test-error-global-init-too-small-array.c: New
	* jit.dg/test-error-global-lvalue-init.c: New
	* jit.dg/test-error-global-nonconst-init.c: New
	* jit.dg/test-global-init-rvalue.c: New
	* jit.dg/test-local-init-rvalue.c: New