[gdb/testsuite] Fix wrong return type in tests

The following tests are marked untested with latest GCC due to a warning
being emitted for a mismatch between their return type and what the lack
of return statement:

* gdb.cp/breakpoint.exp
* gdb.cp/psymtab-parameter.exp
* gdb.cp/shadow.exp

This patch fix the return type to match the function definitions.

2017-11-29  Thomas Preud'homme  <thomas.preudhomme@arm.com>

gdb/testsuite/
	* gdb.cp/breakpoint.cc (bar): Set return type to void.
	* gdb.cp/psymtab-parameter.cc (func): Likewise.
	* gdb.cp/psymtab-parameter.exp: Update comment regarding prototype of
	func ().
	* gdb.cp/shadow.cc (B.func): Return 0.