[mlir] Fix argument attribute attribute reassignment in ConvertStandardToLLVM

The commit switching the calling convention for memrefs (5a177805)
inadvertently introduced a bug in the function argument attribute conversion:
due to incorrect indexing of function arguments it was not assigning the
attributes to the arguments beyond those generated from the first original
argument. This was not caught in the commit since the test suite does have a
test for converting multi-argument functions with argument attributes. Fix the
bug and add relevant tests.