assert that the dtype constants have consistent values on all platforms
assert that the dtype constants have consistent values on all platforms

Summary:
It turns out that macOS, FreeBSD, and Linux all agree on the values
for dtype. If we assume dtype is just the high nibble of mode_t
shifted right by 12, then the Windows CRT agrees too. So hardcode the
values in eden.thrift and add appropriate static_asserts. This opens
the possibility of simply static_cast'ing the numeric values in
Watchman.

Reviewed By: simpkins

Differential Revision: D20975101

fbshipit-source-id: 354ffcbdf3f1d5f8b1715abf1026eaea429d16cf

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