[MSan] Ensure argument shadow initialized on memcpy

We need to explicitly query the shadow here, because it is lazily
initialized for byval arguments. Without opaque pointers this used to
mostly work out, because there would be a bitcast to `i8*` present, and
that would query, and copy in case of byval, the argument shadow.

Reviewed By: vitalybuka, eugenis

Differential Revision: https://reviews.llvm.org/D123602