[llvm-exegesis] InstructionBenchmark::writeYamlTo(): don't forget to flush()

This *APPEARS* to fix a *very* infuriating issue of Yaml's being corrupted,
partially written, truncated. Or at least i'm not seeing the issue
on a new benchmark sweep.

The issue is somewhat rare, happens maybe once in 1000 benchmarks.
Which means there are up to hundreds of broken benchmarks
for a full x86 sweep in a single mode.

llvm-svn: 360124