Function decorate_tiers 

pub fn decorate_tiers(src: &str, pins: &[(&str, &str)]) -> String

Produce a copy of src with a file-level ## Tier <keyword> <value> decorator inserted (just before ## Main, where the parser collects program-level tier pins) for each (keyword, value) pair. The tiering analog of decorate_source: the benchmarks UI injects these to pin an optimization to a tier exactly as it injects ## No <X> to disable one. value is one of eager|t1|t2|t3|never.