[SYCL] Fix a lot of llvm_unreachable("Invalid address space") errors

Currently, CLang doesn't support the generic address space. Sema treats
the generic address space as LangAS::Default:

  /// If this is an OpenCL address space attribute, returns its SYCL
  /// representation in LangAS, otherwise returns default address space.
  LangAS asSYCLLangAS() const {
    switch (getKind()) {
    ...
    case ParsedAttr::AT_OpenCLGenericAddressSpace:
    default:
      return LangAS::Default;
    }
  }

and then this LangAS::Default leads to an invocation of
llvm_unreachable:

    if (ASIdx == LangAS::Default)
      llvm_unreachable("Invalid address space");

As a temporal solution, I've defined `__OPENCL_GENERIC_AS__` as an
empty string for host as well as for device.

Author: Pavel Samolysov

sycl/include/CL/sycl/access/access.hpp

@@ -123,7 +123,7 @@ constexpr bool modeWritesNewData(access::mode m) {
 #define __OPENCL_LOCAL_AS__ __attribute__((opencl_local))
 #define __OPENCL_CONSTANT_AS__ __attribute__((opencl_constant))
 #define __OPENCL_PRIVATE_AS__ __attribute__((opencl_private))
-#define __OPENCL_GENERIC_AS__ __attribute__((opencl_generic))
+#define __OPENCL_GENERIC_AS__
 #else
 #define __OPENCL_GLOBAL_AS__
 #define __OPENCL_GLOBAL_DEVICE_AS__