benchmark

Deprecate constant reference API to DoNotOptimize. (#1493)

The compiler assume that a constant reference, even though escaped via asm
volatile, is unchanged.  The const-ref interface is deprecated to discourage
new uses of it, as subtle compiler optimizations (invariant hoisting, etc.) can
occur.

Within microbenchmarks for Abseil's hashtables, BM_FindMiss_Hot
(https://github.com/google/fleetbench/blob/c0eaa90671d6cc99eb065864e74f0175bee24a5d/fleetbench/swissmap/hot_swissmap_benchmark.cc#L48)
has a `const uint32_t key` is passed to to the lookup of a hashtable.
With the `key` marked `const`, LLVM hoists part of the lookup
calculation outside of the loop.

With the `const` removed, this hoisting does not occur.

Co-authored-by: Dominic Hamon <dominichamon@users.noreply.github.com>
Co-authored-by: dominic <510002+dmah42@users.noreply.github.com>