Reenable "Always import constants" after compile time fixes

Summary:
Reenables importing of constants by default, which was disabled in
D73724 due to excessive thin link times. These inefficiencies were
fixed in D73851.

I re-measured thin link times for a number of binaries that had compile
time explosions with importing of constants previously and confirmed
they no longer have any notable increases with it enabled.

Reviewers: wmi, evgeny777

Subscribers: hiraditya, arphaman, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D74512