Ensure that isNamedCode variable template specializations has internal linkage

Server artefacts compiled with Clang 9 fail to link, due to duplicate definitions of isNamedCode specializations in multiple object files. Per Clang's defect report overview, Clang 9 implements a fix for DR 2387: "Linkage of const-qualified variable template". It appears that variable template specializations now have external linkage unless explicitly marked inline. To fix compile on Clang 9, we should mark isNamedCode as inline.