[mlir] Silence warnings when building with Clang ToT

Previously, these warnings were seen:

[6599/7564] Building native mlir-pdll...
[270/278] Building CXX object tools/mlir/lib/Tools/PDLL/CodeGen/CMakeFiles/obj.MLIRPDLLCodeGen.dir/CPPGen.cpp.obj
In file included from C:/git/llvm-project/mlir/lib/Tools/PDLL/CodeGen/CPPGen.cpp:20:
C:/git/llvm-project/mlir/include\mlir/Tools/PDLL/ODS/Operation.h(202,16): warning: unqualified friend declaration referring to type outside of the nearest enclosing namespace is a Microsoft extension; add a nested name specifier [-Wmicrosoft-unqualified-friend]
  friend class Dialect;
               ^
               ::mlir::
1 warning generated.

[278/278] Linking CXX executable bin\mlir-pdll.exe
[6857/7564] Building CXX object tools/mlir/unittests/ExecutionEngine/CMakeFiles/MLIRExecutionEngineTests.dir/Invoke.cpp.obj
In file included from C:/git/llvm-project/mlir/unittests/ExecutionEngine/Invoke.cpp:20:
C:/git/llvm-project/mlir/include\mlir/ExecutionEngine/MemRefUtils.h(104,27): warning: shift count >= width of type [-Wshift-count-overflow]
  assert(sizeof(T) < (1ul << 32) && "Elemental type overflows");
                          ^  ~~
C:\Program Files (x86)\Windows Kits\10\include\10.0.22000.0\ucrt\assert.h(40,17): note: expanded from macro 'assert'
            (!!(expression)) ||                                                              \
                ^~~~~~~~~~
1 warning generated.