Reset StringMap's NumTombstones on clears and rehashes.

StringMap was not properly updating NumTombstones after a clear or rehash.

This was not fatal until now because the table was growing faster than
NumTombstones could, but with the previous change of preventing infinite
growth of the table the invariant (NumItems + NumTombstones <= NumBuckets)
stopped being observed, causing infinite loops in certain situations.

Patch by José Fonseca!

llvm-svn: 128567