[HLSL] Disable implicit constructors for user-defined structs/classes (#194989)
Disables implicit constructors and assignment operators on user-defined
structs/classes in HLSL.
When a struct is copied or passed as an argument to a function, instead
of using copy constructor the compiler inserts an implicit
_lvalue_-to-_rvalue_ cast where necessary to copy the value. In C++
these implicit casts are not allowed. Sema initialization and
overloading code has been adjusted to enable this code path for HLSL.
For struct in a constant buffer, the implicit cast changes the constant
address space to the default one. Codegen recognized this pattern and
instead of translating the copy as `memcpy`, it copies the struct
element-by-element (because the constant address space struct can have a
different layout).
To efficiently recognize whether a `CXXRecordDecl` is a user-defined
struct/class or if it is an HLSL built-in struct/class, a new bit
`IsHLSLBuiltinRecord` has been added to the `CXXRecordDecl` definition
data. This property is set for struct/classes created in
`HLSLExternalSemaSource`.
Part of #185466
Fixes #153055
llvm-project
8ba6fb1b - [HLSL] Disable implicit constructors for user-defined structs/classes (#194989)
