Mix of namespaces in headers causes build failures

[vladimirlaz]

In the code there is using namespace in headers which causes mixture of namespcaes.
As a C++ developer I wouldn't expect to see using namespace in the C++ code especially in library and/or headers that are used by multiple users.

see intel/llvm-test-suite#659 (comment)

[bader]

@JackAKirk, see intel/llvm-test-suite#666 for more details.
Please, do not forget to enable the test back when you fix the problem.

[JackAKirk]

@bader Apologies for the bug, this issue is addressed in #5217