Add native file for building against LLVM 15 (both Clang and libc++)

scripts/llvm-15.ini

+# For compiling with LLVM 15 (both Clang and libc++)
+# Useful for testing compilation under macOS with Xcode
+# Under Debian, requires the following packages: clang-15 libc++-15-dev libc++abi-15-dev
+
+[binaries]
+cpp = 'clang++-15'
+
+[built-in options]
+# The -fexperimental-library flag is needed to have access to the ranges library
+# Should no longer be needed with LLVM ⩾16.
+# The -Wno-unqualified-std-cast-call remove many warnings about “move” not being
+# qualified with “std::” namespace.
+cpp_args = ['-stdlib=libc++', '-Wno-unqualified-std-cast-call', '-fexperimental-library' ]
+cpp_link_args = [ '-stdlib=libc++' ]