rename folly::detail::getSingletonStackTrace -> folly::symbolizer::getStackTraceStr
rename folly::detail::getSingletonStackTrace -> folly::symbolizer::getStackTraceStr

Summary:
Codemod done with:

```
$ find . -type f -exec sed -i "s#folly/detail/SingletonStackTrace.h#folly/experimental/symbolizer/Symbolizer.h#g" {} +
$ find . -type f -exec sed -i "s#detail::getSingletonStackTrace#symbolizer::getStackTraceStr#g" {} +
$ find . -type f -exec sed -i "s#folly/detail:singleton_stack_trace#folly/experimental/symbolizer:symbolizer#g" {} +
$ find . -type f -exec sed -i "s#:detail_singleton_stack_trace#:experimental_symbolizer_symbolizer#g" {} +
```

Reviewed By: yfeldblum

Differential Revision: D33783971

fbshipit-source-id: 8af88803e69245ad4b5f4f9e729b57930064c3d3

facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst