[C++20] [Modules] Use '-' as the separator of partitions when searching

in filesystems

It is simpler to search for module unit by -fprebuilt-module-path
option. However, the separator ':' of partitions is not friendly.
According to the discussion in https://reviews.llvm.org/D118586, I think
we get consensus to use '-' as the separator instead. The '-' is the
choice of GCC too.

Previously I thought it would be better to add an option. But I feel it
is over-engineering now. Another reason here is that there are too many
options for modules (for clang module mainly) now. Given it is not bad
to use '-' when searching, I think it is acceptable to not add an
option.

Reviewed By: iains

Differential Revision: https://reviews.llvm.org/D120874