Reenable "Always import constants" after compile time fixes

[tejohnson]

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.

[evgeny777]

LGTM

llvm/lib/IR/ModuleSummaryIndex.cpp

STATISTIC(ReadOnlyLiveGVars,
           "Number of live global variables marked read only");
 STATISTIC(WriteOnlyLiveGVars,
           "Number of live global variables marked write only");
 
 static cl::opt<bool> PropagateAttrs("propagate-attrs", cl::init(true),
                                     cl::Hidden,
                                     cl::desc("Propagate attributes in index"));
 
-// FIXME: Enable again when thin link compile time regressions understood and
-// addressed
 static cl::opt<bool> ImportConstantsWithRefs(
-    "import-constants-with-refs", cl::init(false), cl::Hidden,
+    "import-constants-with-refs", cl::init(true), cl::Hidden,
     cl::desc("Import constant global variables with references"));
 
 FunctionSummary FunctionSummary::ExternalNode =
     FunctionSummary::makeDummyFunctionSummary({});
 
 bool ValueInfo::isDSOLocal() const {
   // Need to check all summaries are local in case of hash collisions.
   return getSummaryList().size() &&