[Driver] Allow to collect `-save-stats` data to a file specified in the environment variable.

Using two environment variables `CC_PRINT_INTERNAL_STAT` and
`CC_PRINT_INTERNAL_STAT_FILE` to work like `CC_PRINT_PROC_STAT`.

The purpose of the change is to allow collecting the internal stats
without modifying the build scripts. Write all stats to a single file
to simplify aggregating the data.

Differential Revision: https://reviews.llvm.org/D144981