[NFC][SYCL] Remove explicit cl:: namespace usage

[aelovikov-intel]

That is part of the change of removing cl:: entirely according to the
SYCL 2020 specification.

[v-klochkov]

Please add the link to corresponding changes in llvm-test-suite here.

[aelovikov-intel]

Please add the link to corresponding changes in llvm-test-suite here.

It was supposed to be a standalone change. I'll investigate the failures.

Edit: For some reason I thought multiple tests failed. In fact, it's only one that disables inline in inline namespace cl {, which is an expected fail. The test is being removed in intel/llvm-test-suite#1118.

[aelovikov-intel]

/verify with intel/llvm-test-suite#1118

[aelovikov-intel]

Ping.

[aelovikov-intel]

@intel/llvm-gatekeepers , PR is ready, I think.

[keryell]

Nice.
I do not think this really qualifes for [NFC].
Any program using the old non standard namespace will know fail to compile.

[aelovikov-intel]

This one only changed the references to the symbols and is NFC. Non-NFC elimination has been done later in #6518.

[keryell]

@aelovikov-intel I missed the big picture. Thanks for the clarification!