versioner: fix LLVM assertion failure.

MemoryBuffer::getOpenFileSlice asserts that the map size passed in
isn't -1 (presumably because the offset can be nonzero). Switch to
getOpenFile, which results in an identical call to the implementation
function without the assert.

Bug: http://b/37002637
Test: export FORCE_BUILD_LLVM_COMPONENTS=true
      export FORCE_BUILD_LLVM_DEBUG=true
      export FORCE_BUILD_LLVM_DISABLE_NDEBUG=true
      mma -j && versioner

Change-Id: Ib610db9e07429aa3f64128eaef6ef555c4d1868e