[PTH] Fix data length used for stat cache entries

This came up in a boost build, which apparently uses PTH. This was
broken in r187619 when we migrated it to uses llvm::fs instead of raw
stat calls.

Constructing a test case with a hash table collision in-tree is tough.
Instead, I have a pending change to OnDiskChainedHashTable that asserts
that the reported length of the data agrees with the data actually
written. All of the existing in-tree tests find the bug with this
assert.

llvm-svn: 251828