clang++ and mingw paths start with double slash while compiling

Question

I am trying to use clang++ on a basic hello world project but it cannot find basic headers like iostream. I have already seen multiple questions about that here but nothing has solved my problem. I use Archlinux.

Including: reinstalling mingw, clang17, compiling llvm and mingw-gcc and other things.

However, trying to find what was going on, I ran clang++ -v -target x86_64-w64-mingw32 hello.cpp -o hello.exe and here is the output:

clang version 17.0.6
Target: x86_64-w64-windows-gnu
Thread model: posix
InstalledDir: /sbin
<whole command here>
clang -cc1 version 17.0.6 based upon LLVM 17.0.6 default target x86_64-pc-linux-gnu
ignoring nonexistent directory "//x86_64-w64-mingw32/include/c++"
ignoring nonexistent directory "//x86_64-w64-mingw32/include/c++/x86_64-w64-mingw32"
ignoring nonexistent directory "//x86_64-w64-mingw32/include/c++/backward"
ignoring nonexistent directory "//x86_64-w64-mingw32/include/c++/13.2.0"
ignoring nonexistent directory "//x86_64-w64-mingw32/include/c++/13.2.0/x86_64-w64-mingw32"
ignoring nonexistent directory "//x86_64-w64-mingw32/include/c++/13.2.0/backward"
ignoring nonexistent directory "//include/c++/13.2.0"
ignoring nonexistent directory "//include/c++/13.2.0/x86_64-w64-mingw32"
ignoring nonexistent directory "//include/c++/13.2.0/backward"
ignoring nonexistent directory "//lib/gcc/x86_64-w64-mingw32/13.2.0/include/c++"
ignoring nonexistent directory "//lib/gcc/x86_64-w64-mingw32/13.2.0/include/c++/x86_64-w64-mingw32"
ignoring nonexistent directory "//lib/gcc/x86_64-w64-mingw32/13.2.0/include/c++/backward"
ignoring nonexistent directory "//lib/gcc/x86_64-w64-mingw32/13.2.0/include/g++-v13.2.0"
ignoring nonexistent directory "//lib/gcc/x86_64-w64-mingw32/13.2.0/include/g++-v13.2.0/x86_64-w64-mingw32"
ignoring nonexistent directory "//lib/gcc/x86_64-w64-mingw32/13.2.0/include/g++-v13.2.0/backward"
ignoring nonexistent directory "//lib/gcc/x86_64-w64-mingw32/13.2.0/include/g++-v13.2"
ignoring nonexistent directory "//lib/gcc/x86_64-w64-mingw32/13.2.0/include/g++-v13.2/x86_64-w64-mingw32"
ignoring nonexistent directory "//lib/gcc/x86_64-w64-mingw32/13.2.0/include/g++-v13.2/backward"
ignoring nonexistent directory "//lib/gcc/x86_64-w64-mingw32/13.2.0/include/g++-v13"
ignoring nonexistent directory "//lib/gcc/x86_64-w64-mingw32/13.2.0/include/g++-v13/x86_64-w64-mingw32"
ignoring nonexistent directory "//lib/gcc/x86_64-w64-mingw32/13.2.0/include/g++-v13/backward"
ignoring nonexistent directory "//x86_64-w64-mingw32/include"
ignoring nonexistent directory "//x86_64-w64-mingw32/usr/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/lib/clang/17/include
End of search list.

All the paths start with a double slash which I have never seen. Does anyone know why this might happen? Googling that double slash didn't get me anywhere. What should be between the / is usr.