[mlir][ods] Fix StringRef initialization in builders

For the generated builder taking in unwrapped attribute values,
if the argument is a string, we should avoid wrapping it in quotes;
otherwise we are always setting the string attribute to contain
the string argument's name. The quotes come from StrinAttr's
`constBuilderCall`, which is reasonable for string literals, but
not function arguments containing strings.

Differential Revision: https://reviews.llvm.org/D72977