Fix a major swiftcall ABI bug with trivial C++ class types.

The problem with the previous logic was that there might not be any
explicit copy/move constructor declarations, e.g. if the type is
trivial and we've never type-checked a copy of it.  Relying on Sema's
computation seems much more reliable.

Also, I believe Richard's recommendation is exactly the rule we use
now on the Itanium ABI, modulo the trivial_abi attribute (which this
change of course fixes our handling of in Swift).

This does mean that we have a less portable rule for deciding
indirectness for swiftcall.  I would prefer it if we just applied the
Itanium rule universally under swiftcall, but in the meantime, I need
to fix this bug.

This only arises when defining functions with class-type arguments
in C++, as we do in the Swift runtime.  It doesn't affect normal Swift
operation because we don't import code as C++.

llvm-svn: 328942