[Support/Path] Add path::is_absolute_gnu

[Support/Path] Add path::is_absolute_gnu

Implements IS_ABSOLUTE_PATH from GNU tools.

C++17 is_absolute behavior is different the from the behavior defined by GNU
tools.

According to cppreference.com, C++17 states: "An absolute path is a path
that unambiguously identifies the location of a file without reference
to an additional starting location."

In other words, the rules are:
 1. POSIX style paths with nonempty root directory are absolute.
 2. Windows style paths with nonempty root name and root directory are
    absolute.
 3. No other paths are absolute.

GNU rules are:
 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.

On Windows style the path "C:\Users\Default" has "C:" as root name and "\"
as root directory.

Hence "C:" on Windows is absolute under GNU rules and not absolute under
C++17 because it has no root directory. Likewise "/" and "\" on Windows are
absolute under GNU and are not absolute under C++17 due to empty root name.

Related to PR46368.

Differential Revision: https://reviews.llvm.org/D87667