LLVM-NVPTX+SPIRV-pre-intrinsics

Release for downstream CI for LDC. Only has targets X86, NVPTX and SPIRV.

This is a development release point prior to the conversion of "itanium C++ with extensions mangling" to llvm intrinsics.
While this may suffice for building and using DCompute on OSX (and any other platform other people may provide) it is recommended to build from source to enable receiving updates and bug fixes for LLVM.

Support for SPIRV is, for want of a better term, experimental. This will be periodically updated when upstream breaks compatibility with LDC, the primary consumer of this fork.