edksetup.bat: Simplify the step to use CLANGPDB

edksetup.bat: Simplify the step to use CLANGPDB

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2404

Set the below two environment variables in edksetup.bat:
  set CLANG_HOST_BIN=n
  set CLANG_BIN=C:\Program Files\LLVM\bin\
In Windows, set CLANG_HOST_BIN=n to use nmake command
The CLANG_BIN is only be set if it is not defined.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Ray Ni <ray.ni@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Zhiguang Liu <zhiguang.liu@intel.com>