[C++ Patch] Fixes for duplicate warnings regressions [2/2]

[Paolo Carlini <paolo dot carlini at oracle dot com>]

Hi,

this is another issue, with -Wzero-as-null-pointer-constant, caused (or, 
better, exposed) by the fix for c++/54170: the problem is that 
build_ptrmemfunc, called by cp_convert_to_pointer, itself eventually 
calls cp_convert_to_pointer.

The build_ptrmemfunc bit isn't essential: using the cp_* variant of 
build_c_cast, which takes a tsubst_flags_t, in a function taking a 
tsubst_flags_t seems the right thing to do.

Tested x86_64-linux.

Thanks!
Paolo.

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