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

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."

llvm-svn: 234730