Remove support for truncating names in archives.

* All systems we support have some form of long name support.
* The options has different names and semantics in different implementations
  ('f' on gnu, 'T' on OS X), which makes it unlikely it is normally used on
  build systems.
* It was completely untested.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186078 91177308-0d34-0410-b5e6-96231b3b80d8