[llvm-shlib] Fix windows build failed while llvm non-standalone building.

While build llvm-project as a sub-project on windows, met a build error: 
libllvm-c.exports /llvm/bin\llvm-nm.exe: error: ...builds/rel64ninja/./lib/LLVMDemangle.lib: no such file or directory
The libllvm-c.exports, libllvm-c.args, and lib/*.lib should under LLVM_BINARY_DIR, using CMAKE_BINARY_DIR will cause 'no such file' error while llvm-project built as a sub-project.