ada: Rtsfind should not trash state used in analyzing instantiations.

During analysis of an instantiation, Sem_Ch12 manages formal/actual binding
information in package state (see Sem_Ch12.Generic_Renamings_HTable).
A call to rtsfind can cause another unit to be loaded and compiled.
If this occurs during the analysis of an instantiation, and if the loaded
unit contains a second instantiation, then the Sem_Ch12 state needed for
analyzing the first instantiation can be trashed during the analysis of the
second instantiation. Rtsfind calls that can include the analysis of an
instantiation need to save and restore Sem_Ch12's state.

gcc/ada/

	* sem_ch12.ads: Declare new Instance_Context package, which
	declares a private type Context with operations Save_And_Reset and
	Restore.
	* sem_ch12.adb: Provide body for new Instance_Context package.
	* rtsfind.adb (Load_RTU): Wrap an Instance_Context Save/Restore
	call pair around the call to Semantics.
	* table.ads: Add initial value for Last_Val (because
	Save_And_Reset expects Last_Val to be initialized).