Fix watching structs in C++

Fix watching structs in C++

Some of the watchpoint logic depends on the fact that the head of the
value chain represents the user-specified value to watch.  Thus no
additional values should be added to the value chain after that.  However,
if a watchpoint is defined for a C++ structure/class object, then run-time
type information (RTTI) may be present.  Thus, while constructing the
value chain for the watchpoint, the dynamic type is fetched by
gnuv3_rrti_type, which invokes value_addr, which then adds a new value to
the head of the value chain.  This new value represents the pointer to the
structure instead of the structure itself.

With such a "polluted" value chain the watchpoint logic does not recognize
when the user intended to watch a struct, and can_use_hardware_watchpoint
returns zero.  Instead of a hardware watchpoint, a software watchpoint
will then be set for no apparent reason.

This is fixed by adding an early exit to gnuv3_rtti_type when the input
value is not a dynamic class object.

gdb/testsuite/ChangeLog:

	* gdb.cp/watch-cp.cc: New test.
	* gdb.cp/watch-cp.exp: New file.

gdb/ChangeLog:

	* gnu-v3-abi.c (gnuv3_rtti_type): Add early exit if the given
	value is not a dynamic class object.