[lldb] Avoid FileSystem::Resolve for cached files in the SourceManager

Currently, source files are cached by their resolved path. This means
that before we can query the cache, we potentially have to resolve the
path, which can be slow. This patch avoids the call to FileSystem::Resolve
by caching both the resolved and unresolved path. We now only resolve
the path once when we create and cache a new file.

rdar://110787562

Differential revision: https://reviews.llvm.org/D153726