This change to reflect MLIR's recent addition to LLVM's monorepo. Instead of
stripping llvm from the http archive, use build file map to refer to both MLIR
and LLVM within llvm-project and remove read-only copy that was temporarily
there. Also bump the commit version until after the integration of MLIR into
LLVM monorepo.
PiperOrigin-RevId: 287547194
Change-Id: I4459b96702dc5eb19b58c7ceaa6cabb16f530a43
d6ba353dd9