ThinLTO: do not import function whose linkage prevents inlining.

There is not point in importing a "weak" or a "linkonce" function
since we won't be able to inline it anyway.
We already had a targeted check for WeakAny, this is using the
same check on GlobalValue as the inline, i.e.
isMayBeOverriddenLinkage()

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 268341
diff --git a/llvm/lib/Transforms/IPO/FunctionImport.cpp b/llvm/lib/Transforms/IPO/FunctionImport.cpp
index ec82248..d9860a9 100644
--- a/llvm/lib/Transforms/IPO/FunctionImport.cpp
+++ b/llvm/lib/Transforms/IPO/FunctionImport.cpp
@@ -156,8 +156,8 @@
       CalleeSummaryList,
       [&](const std::unique_ptr<GlobalValueSummary> &SummaryPtr) {
         auto *GVSummary = SummaryPtr.get();
-        if (GlobalValue::isWeakAnyLinkage(GVSummary->linkage()))
-          // There is no point in importing weak symbols, we can't inline them
+        if (GlobalValue::isMayBeOverriddenLinkage(GVSummary->linkage()))
+          // There is no point in importing these, we can't inline them
           return false;
         if (auto *AS = dyn_cast<AliasSummary>(GVSummary)) {
           GVSummary = &AS->getAliasee();