Driver,CodeGen: introduce support for Swift CFString layout

Add a new driver level flag `-fcf-runtime-abi=` that allows one to specify the
runtime ABI for CoreFoundation.  This controls the language interoperability.
In particular, this is relevant for generating the CFConstantString classes
(primarily through the `__builtin___CFStringMakeConstantString` builtin) which
construct a reference to the "CFObject"'s `isa` field.  This type differs
between swift 4.1 and 4.2+.

Valid values for the new option include:
  - objc [default behaviour] - enable ObjectiveC interoperability
  - swift-4.1 - enable interoperability with swift 4.1
  - swift-4.2 - enable interoperability with swift 4.2
  - swift-5.0 - enable interoperability with swift 5.0
  - swift [alias] - target the latest swift ABI

Furthermore, swift 4.2+ changed the layout for the CFString when building
CoreFoundation *without* ObjectiveC interoperability.  In such a case, a field
was added to the CFObject base type changing it from: <{ const int*, int }> to
<{ uintptr_t, uintptr_t, uint64_t }>.

In swift 5.0, the CFString type will be further adjusted to change the length
from a uint32_t on everything but BE LP64 targets to uint64_t.

Note that the default behaviour for clang remains unchanged and the new layout
must be explicitly opted into via `-fcf-runtime-abi=swift*`.

llvm-svn: 345222

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