[CSSPGO] Turn on ext-tsp by default for CSSPGO.

I'm seeing ext-tsp helps CSSPGO for our intern large benchmarks so I'm turning on it for CSSPGO. For non-CS AutoFDO, ext-tsp doesn't seem to help, probably because of lower profile counts quality.

Reviewed By: wenlei

Differential Revision: https://reviews.llvm.org/D119048