[NFCI][SYCL] Simplify SYCL_LANGUAGE_VERSION guards

[aelovikov-intel]

SYCL 1.2 support has been removed quite some time ago, FE only reports 202012L

llvm/clang/lib/Basic/Version.cpp

Lines 129 to 133 in 9e38e3a

	getSYCLVersionMacros(const LangOptions &LangOpts) {
	if (LangOpts.getSYCLVersion() == LangOptions::SYCL_2020)
	return {{"SYCL_LANGUAGE_VERSION", "202012L"}};
	llvm_unreachable("SYCL standard should be set");
	}

so we can expect it's at least that.

[aelovikov-intel]

Looks like we relied on it not being set in two scenarios:

• unittest (easy to fix by setting __SYCL_DISABLE_PARALLEL_FOR_RANGE_ROUNDING__ and SYCL2020_DISABLE_DEPRECATION_WARNINGS)
• custom host compiler (that doesn't seem to have SYCL_LANGUAGE_VERSION)

I'm not sure what the solution for the second one should be, so I'm going to drop this PR for now.