GvFILE() cannot be a pointer to the memory owned by the COP, because

GvFILE() cannot be a pointer to the memory owned by the COP, because
COPs created by use can be freed along this memory, but the GP
remains. Given that several GVs may refer to the same file, use a
shared string rather than an individual allocation per GP.

p4raw-id: //depot/perl@28060