Implements GNU tools IS_ABSOLUTE_PATH.
LLVM's path::is_absolute matches C++17 behavior which is not the same as
GNU tools IS_ABSOLUTE_PATH.
The key differences are:
1) Paths starting with '/' are always absolute;
2) On Windows, paths starting with '\\' are absolute;
3) On Windows, paths starting with drive letter pattern are absolute.
Drive letter pattern example "c:".
Related to PR46368.