Fix potential test failures.

Windows does not allow opened files to be removed. This patch
fixes two types of errors.

 - Output file being the same as input file. Because LLD itself
   holds a file descriptor of the input file, it cannot create an
   output file with the same name as a new file.

 - Removing files before releasing MemoryBuffer objects.

These tests are not failing no because MemoryBuffer happens to
decide not to use mmap on these files. But we shouldn't rely on
that behavior.

llvm-svn: 280507