[Ada] Crash in tagged type constructor with task components

[Ada] Crash in tagged type constructor with task components

2020-06-16  Javier Miranda  <miranda@adacore.com>

gcc/ada/

	* exp_ch6.adb (BIP_Suffix_Kind, Check_BIP_Actuals,
	Is_Build_In_Place_Entity): New subprograms.
	(Make_Build_In_Place_Call_In_Allocator,
	Make_Build_In_Place_Call_In_Anonymous_Context,
	Make_Build_In_Place_Call_In_Assignment,
	Make_Build_In_Place_Call_In_Object_Declaration): Add assertions.
	(Needs_BIP_Task_Actuals): Add missing support for thunks.
	(Expand_Actuals): Ensure that the BIP call has available an
	activation chain and the _master variable.
	* exp_ch9.adb (Find_Enclosing_Context): Initialize the list of
	declarations of empty blocks when the _master variable must be
	declared and the list was not available.