Linalg to LLVM lowering: decrease the reliance on symbol lookup in a module

During the conversion, both the original and the converted function may coexist
in the module and have the same symbol name. There is no guarantee which of the
two will be found by the symbol lookup. Avoid returning the result of the
library function lookup when lowering Linalg to Standard or LLVM. Use the
symbol reference instead. After the conversion completes, only one symbol will
remain and the Ops using SymbolRefAttrs will be referring to the correct one.

PiperOrigin-RevId: 273510079