Starting with C++17, std::string::find accepts anything that can be
converted to std::string_view, including StringRef, allowing us to
avoid creating temporary instances of std::string.

Give the properties from tablegen a `predicate` field that holds the
predicate that the property needs to satisfy, if one exists, and hook
that field up to verifier generation.

Eliminate `llvm::` namespace qualifier for commonly used types in MLIR
TableGen backends to reduce code clutter.

Change `getValueAsListOfDefs` to return a vector of const Record
pointer, and remove `getValueAsListOfConstDefs` that was added as a
transition aid.

This is a part of effort to have better const correctness in TableGen
backends:


https://discourse.llvm.org/t/psa-planned-changes-to-tablegen-getallderiveddefinitions-api-potential-downstream-breakages/81089

This is a part of effort to have better const correctness in TableGen
backends:


https://discourse.llvm.org/t/psa-planned-changes-to-tablegen-getallderiveddefinitions-api-potential-downstream-breakages/81089

Excluded updates to mlir/lib/AsmParser/Parser.cpp ,
which caused LIT failure "FAIL: MLIR::completion.test" on multiple buildbots.

This reverts commit 5a6e52d6ef96d2bcab6dc50bdb369662ff17d2a0.

"FAIL: MLIR::completion.test" on multiple buildbots.

* Strip calls to raw_string_ostream::flush(), which is essentially a no-op
* Strip unneeded calls to raw_string_ostream::str(), to avoid excess indirection.