[MLGO] Use binary protobufs for improved training performance.

[MLGO] Use binary protobufs for improved training performance.

It turns out that during training, the time required to parse the
textual protobuf of a training log is about the same as the time it
takes to compile the module generating that log. Using binary protobufs
instead elides that cost almost completely.

Differential Revision: https://reviews.llvm.org/D106157