Fix non_uniform_work_group narrowing conversion issues

[ellnor01]

Enabling narrowing errors by removing
if(CMAKE_COMPILER_IS_GNUCC OR "${CMAKE_CXX_COMPILER_ID}" MATCHES "(Apple)?Clang")
add_cxx_flag_if_supported(-Wno-narrowing)
endif()
in test_conformance/non_uniform_work_group/CMakeLists.txt produces many errors. A common example is:
test_conformance/non_uniform_work_group/test_advanced_2d.cpp:377:41: error: narrowing conversion of 'primeNumber' from 'int' to 'size_t {aka long unsigned int}' inside { } [-Werror=narrowing]

[ellnor01]

This is a follow up for #787 and so requires #1144 to be merged first