Revert r234620 and others, "Actually check if lseek works instead of using a filename...

Revert r234620 and others, "Actually check if lseek works instead of using a filename based heuristic." It was affected by r234615, which was reverted in r234721.

  r234620, "Actually check if lseek works instead of using a filename based heuristic."
  r234621, "Testcase for the previous commit."
  r234718, "Suppress clang/test/PCH/emit-pth.c on win32, for now while investigating."

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@234730 91177308-0d34-0410-b5e6-96231b3b80d8