The attached patch fixes some more places in c/c-parser.c where the
src_range field of a c_expr was being left uninitialized, this time for
various Objective C constructs.

The source ranges are verified using the same unit-testing plugin used
for C expressions.  This leads to a wart, which is that it means having
a .m test file lurking below gcc.dg/plugin.  A workaround would be to
create an objc.dg/plugin subdirectory, with a new plugin.exp for testing
Objective C plugins, and having it reference the existing plugin below
gcc.dg/plugin.  That seemed like excessive complication, so I went for
the simpler approach of simply putting the .m file below gcc.dg/plugin.

I manually verified that this fixes the specific valgrind warnings
reported in the PR, and visually inspected the code for
objective-c-specific instances of uninitialized c_expr src_range values.

Bootstrapped&regrtested on x86_64-pc-linux-gnu; adds 15 PASS results to
gcc.sum.

OK for trunk?