Simplify shims

[lorentey]

Now that we can rely on https://reviews.llvm.org/D86218, we don’t need to tiptoe around the use of _Atomic(struct _sa_dword).

Rip out the clumsy encode/decode mess and the use of __uint128_t.

As a side effect of removing the use of __uint128_t, this PR deprecates the DoubleWord.low and DoubleWord.high properties (and the associated initializer), replacing them with first and second.

Checklist

• I've read the Contribution Guidelines
• My contributions are licensed under the Swift license.
• I've followed the coding style of the rest of the project.
• I've added tests covering all new code paths my change adds to the project (if appropriate).
• I've verified that my change does not break any existing tests.
• I've updated the documentation if necessary.

[lorentey]

@swift-ci test

[lorentey]

@swift-ci test

[lorentey]

@swift-ci test

[lorentey]

@swift-ci test

[stephentyrone]

deprecating high/low fills me with joy.