Always re-create the Clang resource directory when building.

This makes sure that the copy is always done correctly, due to the Premake bug.

build/LLVM.lua

@@ -94,6 +94,7 @@ function CopyClangIncludes()
     { 
         -- Workaround Premake OS-specific behaviour when copying folders:
         -- See: https://github.com/premake/premake-core/issues/1232
+        '{RMDIR} "%%{cfg.buildtarget.directory}/lib"',
         '{MKDIR} "%%{cfg.buildtarget.directory}/lib"',
         string.format('{COPY} "%s" "%%{cfg.buildtarget.directory}/lib"', clangBuiltinIncludeDir) 
     }