[dsymutil] Fix assertion in the Reproducer/FileCollector when TMPDIR is empty

Fix a assertion in dsymutil coming from the Reproducer/FileCollector.
When TMPDIR is empty, the root becomes a relative path, triggering an
assertion when adding a relative path to the VFS mapping. This patch
fixes the issue by resolving the relative path and also moves the
assertion up to make it easier to diagnose these issues in the future.

rdar://102170986

Differential revision: https://reviews.llvm.org/D137959