Revert "[clang-cl] Always interpret the LIB env var as separated with semicolons"

This reverts commit 4d85444b.

This commit broke building lldb's NativeProcessProtocolTest.cpp,
with errors like these:

In file included from include/llvm/Support/Process.h:32:0,
                 from tools/lldb/unittests/Host/NativeProcessProtocolTest.cpp:12:
include/llvm/Support/Program.h:39:11: error: reference to ‘pid_t’ is ambiguous
   typedef pid_t procid_t;

/usr/include/sched.h:38:17: note: candidates are: typedef __pid_t pid_t
 typedef __pid_t pid_t;

tools/lldb/include/lldb/lldb-types.h:85:18: note: typedef uint64_t lldb::pid_t
 typedef uint64_t pid_t;