[IROutliner] Adding consistent function attribute merging

When combining extracted functions, they may have different function
attributes. We want to make sure that we do not make any assumptions,
or lose any information. This attempts to make sure that we consolidate
function attributes to their most general case.

Tests:
llvm/test/Transforms/IROutliner/outlining-compatible-and-attribute-transfer.ll
llvm/test/Transforms/IROutliner/outlining-compatible-or-attribute-transfer.ll

Reviewers: jdoefert, paquette

Differential Revision: https://reviews.llvm.org/D87301