[mlir] Add function_entry_count to LLVMFuncOp

This commit introduces the function_entry_count metadata field to the
LLVMFuncOp and adds both the corresponding import and export
funtionalities.
The import of the function metadata uses the same infrastructure as the
instruction metadata, i.e., it dispatches through a dialect interface.

Reviewed By: gysit

Differential Revision: https://reviews.llvm.org/D141001