[XRay] Refactor loadTraceFile(...) into two (NFC)

This patch splits the file trace loading function into two versions, one
that takes a filename and one that takes a `DataExtractor`.

This change is a precursor to larger changes to increase test coverage
for the trace loading implementation.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@340603 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed