[lldb] [POSIX-DYLD] Update the cached exe path after attach

Fix the POSIX-DYLD plugin to update the cached executable path after
attaching.  Previously, the path was cached in DYLDRendezvous
constructor and not updated afterwards.  This meant that if LLDB was
attaching to a process (e.g. via connecting to lldb-server), the code
stored the empty path before DidAttach() resolved it.  The fix updates
the cached path in DidAttach().

This fixes a new instance of https://llvm.org/pr17880

Differential Revision: https://reviews.llvm.org/D92264