[SYCL] Remove <iostream> from header and source files

[raaiq1]

-Removed <iostream> headers from header and source files to best conform to LLVM Coding Standards : https://llvm.org/docs/CodingStandards.html#include-iostream-is-forbidden .

-Substituted the removed headers with a proxy header which uses external linkage to resolve symbols in <iostream>

[aelovikov-intel]

Can you please add the following tests:

1. Compile just #include <sycl/sycl.hpp>, ensure that global objects aren't created (i.e. resulting obj file is empty).
2. The following compiles cleanly

#include <iostream>
#include <sycl/sycl.hpp>
void foo() { std::cout << 42; }

3. The following compiles cleanly

#include <sycl/sycl.hpp>
#include <iostream>
void foo() { std::cout << 42; }

[raaiq1]

Can you please add the following tests:

1. Compile just #include <sycl/sycl.hpp>, ensure that global objects aren't created (i.e. resulting obj file is empty).
2. The following compiles cleanly

#include <iostream>
#include <sycl/sycl.hpp>
void foo() { std::cout << 42; }

3. The following compiles cleanly

#include <sycl/sycl.hpp>
#include <iostream>
void foo() { std::cout << 42; }

I'm sorry, I don't understand what you mean by 'compile cleanly'

[aelovikov-intel]

That clang -fsycl -c foo.cpp finishes without warnings/errors.

[cperkinsintel]

LGTM!!

[cperkinsintel]

failing CUDA test is known to be flaky. ( #8800 )

[raaiq1]

@intel/llvm-gatekeepers slight ping to merge commit. Thanks

[pvchupin]

@raaiq1, please follow up on windows post commit issue: https://github.com/intel/llvm/runs/7587694915?check_suite_focus=true

[aelovikov-intel]

I'm working on the build failure as @raaiq1 is on holiday today.

Edit: #6506