[libc++][test] Speed up input generating functions for benchmark tests (#115544)

The input generating functions for benchmark tests in the GenerateInput.h
file can be slightly improved by invoking vector::reserve before calling
vector::push_back. This slight performance improvement could potentially
speed-up all benchmark tests for containers and algorithms that use these
functions as inputs.
GitOrigin-RevId: c25e09e238c6f872a116d10bbefba0beff145a57
1 file changed