The patch introduce __builtin_spirv_generic_cast_to_ptr_explicit which
is lowered to the llvm.spv.generic.cast.to.ptr.explicit intrinsic.

The SPIR-V builtins are now split into 3 differents file:
BuiltinsSPIRVCore.td,
BuiltinsSPIRVVK.td for Vulkan specific builtins, BuiltinsSPIRVCL.td for
OpenCL specific builtins
and BuiltinsSPIRVCommon.td for common ones.

The patch also introduces a new header defining its SPIR-V friendly
equivalent (__spirv_GenericCastToPtrExplicit_ToGlobal,
__spirv_GenericCastToPtrExplicit_ToLocal and
__spirv_GenericCastToPtrExplicit_ToPrivate). The functions are declared
as aliases to the new builtin allowing C-like languages to have a
definition to rely on as well as gaining proper front-end diagnostics.

The motivation for the header is to provide a stable binding for
applications or library (such as SYCL) and allows non SPIR-V targets to
provide an implementation (via libclc or similar to how it is done for
gpuintrin.h).

Resolves #99114.

Tasks completed:
- Implement `faceforward` in
`hlsl_intrinsics.h`/`hlsl_intrinsic_helpers.h`
- Implement `faceforward` SPIR-V target builtin in
`clang/include/clang/Basic/BuiltinsSPIRV.td`
- Add a SPIR-V fast path in `hlsl_intrinsic_helpers.h`
- Add sema checks for `faceforward` to `CheckSPIRVBuiltinFunctionCall`
in `clang/lib/Sema/SemaSPIRV.cpp`
- Add codegen for SPIR-V `faceforward` builtin to `EmitSPIRVBuiltinExpr`
in `SPIR.cpp`
- Add HLSL codegen tests to
`clang/test/CodeGenHLSL/builtins/faceforward.hlsl`
- Add SPIRV builtin codegen tests to
`clang/test/CodeGenSPIRV/Builtins/faceforward.c`
- Add sema tests to
`clang/test/SemaHLSL/BuiltIns/faceforward-errors.hlsl`
- Add spirv sema tests to
`clang/test/SemaSPIRV/BuiltIns/faceforward-errors.c`
- Create the `int_spv_faceforward` intrinsic in `IntrinsicsSPIRV.td`
- In `SPIRVInstructionSelector.cpp` create the `faceforward` lowering
and map it to `int_spv_faceforward` in
`SPIRVInstructionSelector::selectIntrinsic`
- Create SPIR-V backend test case in
`llvm/test/CodeGen/SPIRV/hlsl-intrinsics/faceforward.ll`

Incomplete tasks:
- Create SPIR-V backend test case in
`llvm/test/CodeGen/SPIRV/opencl/faceforward.ll`
- Not applicable because the OpenCL SPIR-V extended instruction set does
not include a `faceforward` function

Follow-up tasks:
- Implement pattern matching for `faceforward` in `SPIRVCombine.td` and
`SPIRVPreLegalizerCombiner.cpp`
- In `faceforward.ll`, change `--target-env spv1.4` to `vulkan1.3` and
update the test accordingly once
[#136344](https://github.com/llvm/llvm-project/issues/136344) has been
resolved

Closes #99156.


Tasks completed:
- Implement `smoothstep` using HLSL source in `hlsl_intrinsics.h`
- Implement the `smoothstep` SPIR-V target built-in in
`clang/include/clang/Basic/BuiltinsSPIRV.td`
- Add sema checks for `smoothstep` to `CheckSPIRVBuiltinFunctionCall` in
`clang/lib/Sema/SemaSPIRV.cpp`
- Add codegen for spv `smoothstep` to `EmitSPIRVBuiltinExpr` in
`clang/lib/CodeGen/TargetBuiltins/SPIR.cpp`
- Add codegen tests to `clang/test/CodeGenHLSL/builtins/smoothstep.hlsl`
- Add spv codegen test to
`clang/test/CodeGenSPIRV/Builtins/smoothstep.c`
- Add sema tests to
`clang/test/SemaHLSL/BuiltIns/smoothstep-errors.hlsl`
- Add spv sema tests to
`clang/test/SemaSPIRV/BuiltIns/smoothstep-errors.c`
- Create the `int_spv_smoothstep` intrinsic in `IntrinsicsSPIRV.td`
- In SPIRVInstructionSelector.cpp create the `smoothstep` lowering and
map it to `int_spv_smoothstep` in
`SPIRVInstructionSelector::selectIntrinsic`
- Create SPIR-V backend test case in
`llvm/test/CodeGen/SPIRV/hlsl-intrinsics/smoothstep.ll`
- Create SPIR-V backend test case in
`llvm/test/CodeGen/SPIRV/opencl/smoothstep.ll`

Remove superfluous header files after refactor in #132252

clang/lib/CodeGen/CGBuiltin.cpp is over 1MB long (>23k LoC), and can
take minutes to recompile (depending on compiler and host system) when
modified, and 5 seconds for clangd to update for every edit. Splitting
this file was discussed in this thread:

   https://discourse.llvm.org/t/splitting-clang-s-cgbuiltin-cpp-over-23k-lines-long-takes-1min-to-compile/

and the idea has received a number of +1 votes, hence this change.