[Graph Writer] Limit the length of the graph name because Windows can't handle it.

Windows can't handle paths longer than 260 code points without \\?\. Even
with \\?\ it can't handle path components longer than 255 code points. So
limit graph names to the arbitrary length of 140. Random characters are still
added to the end, so it's ok if graph names collide.

Differential Revision: http://reviews.llvm.org/D3883

llvm-svn: 209483