The "desired result type" code in the expression

parser has hitherto been an implementation waiting
for a use.  I have now tied the '-o' option for
the expression command -- which indicates that the
result is an Objective-C object and needs to be
printed -- to the ExpressionParser, which
communicates the desired type to Clang.

Now, if the result of an expression is determined
by an Objective-C method call for which there is
no type information, that result is implicitly
cast to id if and only if the -o option is passed
to the expression command.  (Otherwise if there
is no explicit cast Clang will issue an error.
This behavior is identical to what happened before
r146756.)

Also added a testcase for -o enabled and disabled.

llvm-svn: 147099