[HLSL] cbuffer: create host layout structs (#122820)
Creates layout struct for `cbuffer` in Sema which will contains only
declarations contributing to the constant buffer layout. Anything else
will be filtered out, such as static variables decls, struct and
function definitions, resources, or empty struct and zero-sized arrays.
If the constant buffer includes a struct that contains any of the above
undesirable declarations, a new version of this struct should be created
with these declarations filtered out as well.
The definition of buffer layout struct will be added to the
HLSLBufferDecl AST node as the last node. Any layout structs for
embedded structures will be added there as well.
Fixes #122553
