[Driver] Don't open files multiple times.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@178268 91177308-0d34-0410-b5e6-96231b3b80d8
[ELF] Cache contentType and permissions.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@178269 91177308-0d34-0410-b5e6-96231b3b80d8