[PECOFF][Driver] Show error message if no input file is given.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@186882 91177308-0d34-0410-b5e6-96231b3b80d8
Fix -Wsign-compare warnings.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@186884 91177308-0d34-0410-b5e6-96231b3b80d8