Runtime: correct invalid C++ workaround

[compnerd]

The placement new operator is only available when <new> has been
included. Add the missing include to the header to allow us to get the
definition of the placement new allocator. This allows us to remove the
workaround that was there for Windows, and should hopefully repair the
Android build as well.

Thanks to @grynspan for helping identify the underlying issue!

Replace this paragraph with a description of your changes and rationale. Provide links to external references/discussions if appropriate.

Resolves SR-NNNN.

[compnerd]

@swift-ci please test

[finagolfin]

This pull seems to make no difference when targeting Android armv7:

In file included from /home/butta/swift/stdlib/public/runtime/KnownMetadata.cpp:21:
/home/butta/swift/stdlib/public/runtime/MetadataImpl.h:103:12: error: no matching function for call to 'operator new'
    return new (dest) T(*src);

[compnerd]

Ugh, okay, so reverting the change is not going to help as you mentioned - the underlying problem is that there is a fundamental allocator mismatch that we are not accounting for. We need to figure out how to fix the 32-bit platforms that are not guaranteeing 16-byte alignment (or does android guarantee that? in which case, we have a very simple fix)

[finagolfin]

does android guarantee that?

Unsure, I don't use armv7 much.

[compnerd]

Okay, verified that Android does not guarantee the required 16-byte alignment (you are only 8-byte aligned on 32-bit which is a problem). Although the ARMv7 builds are currently broken, it is a real issue that this is uncovering. I think that we should repair it forward rather than reverting the change as a result.

[compnerd]

Let's continue the android discussion at #42347 and let this one through to improve the runtime.