diagnostics: convert diagnostic_t to enum class diagnostics::kind: restore build...

diagnostics: convert diagnostic_t to enum class diagnostics::kind: restore build with GCC 5

Building with GCC 5, with commit 3cc27ed5b6fb4253d3fe2139dcc6295e85372a3a
"diagnostics: convert diagnostic_t to enum class diagnostics::kind", we got:

    ../../source-gcc/gcc/diagnostics/option-classifier.cc: In member function ‘diagnostics::kind diagnostics::option_classifier::update_effective_level_from_pragmas(diagnostics::diagnostic_info*) const’:
    ../../source-gcc/gcc/diagnostics/option-classifier.cc:212:20: error: ‘kind’ is not a class, namespace, or enumeration
            if (kind != kind::unspecified)
                        ^
    make[2]: *** [Makefile:1212: diagnostics/option-classifier.o] Error 1

Resolve this similar to, for example,
commit r14-4521-g08d0f840dc7ad212ab75d094373b01cbfc004e67
"analyzer: fix build with gcc < 6".

	gcc/
	* diagnostics/option-classifier.cc
	(update_effective_level_from_pragmas): In presence of local
	variable 'kind', explicitly state 'diagnostics::' scope for 'kind'
	enum.