[ORC] Skip ST_File symbols in MaterializationUnit interfaces / resolution.

ST_File symbols aren't relevant for linking purposes, but can end up shadowing
real symbols if they're not filtered.

No test case yet: The ideal testcase for this would be an ELF llvm-jitlink test,
but llvm-jitlink support for ELF is still under development. We should add a
testcase for this once support lands in tree.