Add a new langopt NativeInt16Type to control support for 16 bit
integers.
Enable by default for all languages but HLSL. 
HLSL defines uint16_t and int16_t as a typedef of short. If
-enable-16bit-types is not used, the typedefs don't exist so int16_t and
uint16_t can't be used. However, short was still allowed. This change
will produce an error 'unknown type name short' if -enable-16bit-types
isn't used.
Update failing tests. 
Add new test.
Closes #81779

Adds support for elementwise and aggregate splat casting struct types
with bitfields. Replacing existing Flattening function which used to
produce a list of GEPs representing a flattened object with one that
produces a list of LValues representing a flattened object. The LValues
can be used by EmitStoreThroughLValue and EmitLoadOfLValue, ensuring
bitfields are properly loaded and stored. This also simplifies the code
in the elementwise and aggregate splat casting functions.
Closes #125986

Fixes #121010.

Implement HLSLElementwiseCast excluding support for splat cases
Do not support casting types that contain bitfields.
Partly closes https://github.com/llvm/llvm-project/issues/100609 and
partly closes https://github.com/llvm/llvm-project/issues/100619
Re-land #118842 after fixing warning as an error, found by a buildbot.

Reverts llvm/llvm-project#118842

Implement HLSLElementwiseCast excluding support for splat cases
Do not support casting types that contain bitfields.
Partly closes #100609 and partly closes #100619