[Support/Path] Add path::is_gnu_style_absolute
Implements GNU tools IS_ABSOLUTE_PATH.
LLVM's path::is_absolute behavior is not the same as GNU tools
GNU style absolute defines the following rules:
1. Paths starting with a path separator are absolute.
2. Windows style paths are also absolute if they start with a character
followed by ':'.
3. No other paths are absolute.
Related to PR46368.