[Support/Path] sys::path::replace_path_prefix fix and simplifications

Added unit tests for 2 scenarios that were failing.
Made replace_path_prefix back to 3 parameters instead of 5, simplifying the implementation. The other 2 were always used with the default value.

This commit is intended to be the first of 3:
1) simplify/fix replace_path_prefix.
2) use it in the context of -fdebug-prefix-map and -fmacro-prefix-map (see D76869).
3) Make Windows version of replace_path_prefix insensitive to both case and separators (slash vs backslash).

Differential Revision: https://reviews.llvm.org/D77223