Don't import variadic functions

Summary:
This patch adds IsVariadicFunction bit to summary in order
to not import variadic functions. Inliner doesn't inline
variadic functions because it is hard to reason about it.

This one small fix improves Importer by about 16%
(going from 86% to 100% of imported functions that are
inlined anywhere)
on some spec benchmarks like 'int' and others.

Reviewers: eraman, mehdi_amini, tejohnson

Subscribers: mehdi_amini, llvm-commits

Differential Revision: https://reviews.llvm.org/D23339

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@278432 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Bitcode/Writer/BitcodeWriter.cpp

@@ -991,7 +991,7 @@ static uint64_t getEncodedGVSummaryFlags(GlobalValueSummary::GVFlags Flags) {
   uint64_t RawFlags = 0;
 
   RawFlags |= Flags.HasSection; // bool
-
+  RawFlags |= (Flags.IsNotViableToInline << 1);
   // Linkage don't need to be remapped at that time for the summary. Any future
   // change to the getEncodedLinkage() function will need to be taken into
   // account here as well.