[mlir] Relax restriction on name location parsing

We currently restrict parsing of location to not allow nameloc being
nested inside nameloc. This restriction may be historical as there
doesn't seem to be a reason for it anymore (locations like this can be
constructed in C++ and they print fine). Relax this restriction in the
parser to allow this nesting.

Differential Revision: https://reviews.llvm.org/D115581