CodeGen: ensure 8-byte aligned String Swift CF ABI

[compnerd]

CFStrings should be 8-byte aligned when built for the Swift CF runtime
ABI as the atomic CF info field must be properly aligned. This is a
problem on 32-bit platforms which would give the structure 4-byte
alignment rather than 8-byte alignment.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@368471 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit ee5531b)

[compnerd]

CC: @dexonsmith @millenomi

This is needed to repair Foundation on non-Darwin 32-bit targets. Since this is controlled by the -fcf-runtime-abi= flag, with the default being ObjC, this has no impact on the Darwin path as Darwin targets do not currently use the Swift runtime ABI (due to historical accidents). Additionally, this patch only changes things on 32-bit targets (watchOS is excluded as it is a Darwin target), as 64-bit targets would use sizeof(void *) as the alignment (which would be 8-bytes). Even there, it is increasing the alignment (4 -> 8) and should be safe for interoperability (even though there is not ABI guarantees yet on non-Darwin targets).

[dexonsmith]

LGTM.