Use a sorted array instead of a map to store AttrBuilder string attributes

Using and std::map<SmallString, SmallString> for target dependent attributes is
inefficient: it makes its constructor slightly heavier, and involves extra
allocation for each new string attribute. Storing the attribute key/value as
strings implies extra allocation/copy step.

Use a sorted vector instead. Given the low number of attributes generally
involved, this is cheaper, as showcased by

https://llvm-compile-time-tracker.com/compare.php?from=5de322295f4ade692dc4f1823ae4450ad3c48af2&to=05bc480bf641a9e3b466619af43a2d123ee3f71d&stat=instructions

Differential Revision: https://reviews.llvm.org/D116599