Make LLVM command-line tools overwrite their output files without -f.

Make LLVM command-line tools overwrite their output files without -f.
This is conventional command-line tool behavior. -f now just means
"enable binary output on terminals".

Add a -f option to llvm-extract and llvm-link, for consistency.

Remove F_Force from raw_fd_ostream and enable overwriting and
truncating by default. Introduce an F_Excl flag to permit users to
enable a failure when the file already exists. This flag is
currently unused.

Update Makefiles and documentation accordingly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@79990 91177308-0d34-0410-b5e6-96231b3b80d8