fixes #165663

The bug was that we were using the initalizer lists index to populate
the matrix. This meant that [0..n] would coorelate to [0..n] indicies of
the flattened matrix. Hence why we were seeing the Row-major order: [ 0
1 2 3 4 5 ]. To fix this we can simply converted these indicies to the
Column-major order: [ 0 3 1 4 2 5 ].

The net effect of this is the layout of the matrix is now correct and we
don't need to change the MatrixSubscriptExpr indexing scheme.

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Co-authored-by: Deric C. <cheung.deric@gmail.com>
Co-authored-by: Helena Kotas <hekotas@microsoft.com>

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

This simplifies and cleans up the vector list initialization behavior.
This simplifies the work we do in SemaInit by just relying on SemaHLSL's
initialization list flattening. This change fixes some outstanding
limitations by supporting structure to vector initialization, but
re-introduces HLSL's limitations around overload resolution in
initializers.

---------

Co-authored-by: Helena Kotas <hekotas@microsoft.com>

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

Enable InitList code to handle zero sized structs; this includes structs
with no fields and those with only unnamed bitfields.
Closes #157920

In HLSL Init List code avoid initializing unnamed bitfields and avoid
initializing using unnamed bit fields.
Add tests.
Closes #157922

Fixes #148063 by preventing the ByVal attribute from being placed on out
and inout function parameters which causes them to be eliminated by the
Dead Store Elimination (DSE) pass.

Implements

https://github.com/llvm/wg-hlsl/blob/main/proposals/0026-symbol-visibility.md.

The change is to stop using the `hlsl.export` attribute. Instead,
symbols with "program linkage" in HLSL will have export linkage with
default visibility, and symbols with "external linkage" in HLSL will
have export linkage with hidden visibility.

When an HLSL Init list is producing a Scalar, handle OpaqueValueExprs in
the Init List with 'emitInitListOpaqueValues'
Copied from 'AggExprEmitter::VisitCXXParenListOrInitListExpr'
Closes #136408

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Co-authored-by: Chris B <beanz@abolishcrlf.org>

Fixes #121010.