Windows: support `DoLoadImage`

This implements `DoLoadImage` and `UnloadImage` in the Windows platform
plugin modelled after the POSIX platform plugin.  This was previously
unimplemented and resulted in a difficult to decipher error without any
logging.

This implementation is intended to support enables the use of LLDB's
Swift REPL on Windows.

Paths which are added to the library search path are persistent and
applied to all subsequent loads.  This can be adjusted in the future by
storing all the cookies and restoring the path prior to returning from
the helper.  However, the dynamic path count makes this a bit more
challenging.

Reviewed By: @JDevlieghere
Differential Revision: https://reviews.llvm.org/D77287