dependencies/llvm: Fixup bad output from llvm-config on windows
dependencies/llvm: Fixup bad output from llvm-config on windows

It turns out that llvm-config on windows can return such wonderful
output as `-LIBDIR:c:\\... c:\\abslute\\path\\to\\lib.lib`, which was
all fine and dandy when we were blindly passing it through, GCC/MinGW
ignored it and MSVC understood it meant `/LIBDIR:`; however, after we
added some code to validate linker arguments, and we have some code
before the validation that tries to remove posix style -L arguments,
resulting in IBDIR:..., which doesn't validate.

This patch fixes up the output of llvm-config so that -LIBDIR: is
replaced by the the link libdir argument of the compiler, via the
compiler/linker method for getting that.

Fixes #5419

pexip/meson,becm/meson,mesonbuild/meson,QuLogic/meson,QuLogic/meson,jpakkane/meson,becm/meson,mesonbuild/meson,pexip/meson,pexip/meson,becm/meson,jpakkane/meson,pexip/meson,mesonbuild/meson,jpakkane/meson,jpakkane/meson,QuLogic/meson,QuLogic/meson,QuLogic/meson,QuLogic/meson,mesonbuild/meson,pexip/meson,becm/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,jpakkane/meson,jpakkane/meson,jpakkane/meson,becm/meson,QuLogic/meson,becm/meson,mesonbuild/meson,becm/meson,QuLogic/meson,mesonbuild/meson,pexip/meson,becm/meson,pexip/meson,jpakkane/meson,becm/meson,jpakkane/meson,mesonbuild/meson,pexip/meson,becm/meson,QuLogic/meson,pexip/meson,mesonbuild/meson