Rename FileToMutable -> SimpleFileWrapper.

FileToMutable is what this class does, but this class (or, to be precise,
an instance of this class) is a wrapper of the other SimpleFile. It's odd
that the class was named like a function.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@210089 91177308-0d34-0410-b5e6-96231b3b80d8