The two forms of placement operator new defined in <new> return their
pointer argument and may not be displaced by user-defined functions.
But because they are ordinary (not built-in) functions this property
isn't reflected in their declarations alone, and there's no user-
level attribute to annotate them with.  When they are inlined
the property is transparent in the IL but when they are not (without
inlining such as -O0), calls to the operators appear in the IL and
cause -Wmismatched-new-delete to try to match them with the functions
called to deallocate memory.  When the pointer to the memory was
obtained from a function that matches the deallocator but not
the placement new, the warning falsely triggers.

The attached patch solves this by detecting calls to placement new
and treating them the same as those to other pass-through calls (such
as memset).  In addition, it also teaches -Wfree-nonheap-object about
placement delete, for a similar reason as above.  Finally, it also
adds a test for attribute fn spec indicating a function returns its
argument.  It's not necessary for the fix (I had initially though
placement new might have the attribute) but it seems appropriate
to check.

Tested on x86_64-linux.

Martin