[mlir] move _mlir_alloc and friends to CRunnerUtils

These functions don't depend on the C++ runtime and therefore belong to
CRunnerUtils. Clean up the macros on the way as `_MSC_VER` indicates the
compiler, not the platform, which is indicated by `_WIN32` and will be
present when, e.g., compiling with minGW.

Reviewed By: rdzhabarov

Differential Revision: https://reviews.llvm.org/D130025