[SYCL] compiler issues an error when std::nullptr_t is used as a kernel name type

[kbobrovs]

The following test fails to compile:
null_name.cpp.txt

$ clang++ -fsycl null_name.cpp -lOpenCL
In file included from :1:
/tmp/null_name-36241f.h:41:31: error: unknown type name 'nullptr_t'; did you mean 'std::nullptr_t'?
template <> struct KernelInfo<nullptr_t> {
^~~~~~~~~
std::nullptr_t
/usr/lib/gcc/x86_64-linux-gnu/7.3.0/../../../../include/x86_64-linux-gnu/c++/7.3.0/bits/c++config.h:235:29: note: 'std::nullptr_t' declared here
typedef decltype(nullptr) nullptr_t;
^
1 error generated.

[erichkeane]

@premanandrao , is this one no longer valid?

[premanandrao]

@premanandrao , is this one no longer valid?

I gave this one to @jtmott-intel for the cleanup that @rolandschulz requested.

[jtmott-intel]

Resolved by #1963.

We still issue an error when std::nullptr_t is used as a kernel name, it's just that now we do so intentionally instead of accidentally. Kernel names are expected to be forward declarable, and the C++ standard forbids any name in the std namespace to be forward declared. Therefore, std::nullptr_t -- or std::anything -- may not be used in a kernel name, and if it is, then we emit a diagnostic error, "Kernel name cannot be a type in the std namespace."