Changes

[MLIR] Use memref.copy ops in BufferResultsToOutParams pass.

Both copy/alloc ops are using memref dialect after this change.

Reviewed By: silvas, mehdi_amini

Differential Revision: https://reviews.llvm.org/D109480

Make the --mlir-disable-threading command line option overrides the C++ API usage

This seems in-line with the intent and how we build tools around it.
Update the description for the flag accordingly.
Also use an injected thread pool in MLIROptMain, now we will create
threads up-front and reuse them across split buffers.

Differential Revision: https://reviews.llvm.org/D109802

Revert "[flang] Make 'this_image()' an intrinsic function"

This reverts commit 81f8ad1769665a569a235b749e0e9e69ce7dc65e.
This seems to break the shared libs build
(linaro-flang-aarch64-sharedlibs bot) with:

  undefined reference to `Fortran::semantics::IsCoarray(Fortran::semantics::Symbol const&)

(from tools/flang/lib/Evaluate/CMakeFiles/obj.FortranEvaluate.dir/tools.cpp.o)

When linking lib/libFortranEvaluate.so.14git