__builtin_FILE() is inconsistent on path separator on Windows

[SuperWig]

Bugzilla Link	45731
Version	10.0
OS	Windows NT
CC	@AaronBallman,@amccarth-google,@zygoloid,@rnk

Extended Description

Given the following

src/main.cpp
src/header.h

And the following code:

header.h:

#include <iostream>

void test()
{
    std::cout << __builtin_FILE() << '\n';
}

main.cpp:

#include "header.h"

int main()
{
    test();
    std::cout << __builtin_FILE() << '\n';
}

I get the following output

src/header.h
src\main.cpp

Discovered this when implementing std::source_location for Microsoft here

microsoft/STL#664 (comment)

[rnk]

Clang internally does as little path canonicalization as possible, although I've forgotten precisely why, and this behavior is a reflection of that. __FILE__ has the same issue. This results in user-supplied paths often containing backslashes (-IC:\my~8.3\sdk\path) being concatenated with portable separators (forward slashes) and traversal components (..). It has long been suggested that this is not what users want to see in __FILE__, but changing __FILE__ is of course fraught with peril.