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

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@360124 91177308-0d34-0410-b5e6-96231b3b80d8

GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm