[ThinLTO] Add funtions in callees metadata to CallGraphEdges

Summary:
If there's a callees metadata attached to the indirect call instruction, add CallGraphEdges to the callees mentioned in the metadata when computing FunctionSummary.

* Why this is necessary:
Consider following code example:
```
(foo.c)
static int f1(int x) {...}
static int f2(int x);
static int (*fptr)(int) = f2;
static int f2(int x) {
  if (x) fptr=f1; return f1(x);
}
int foo(int x) {
  (*fptr)(x); // !callees metadata of !{i32 (i32)* @f1, i32 (i32)* @f2} would be attached to this call.
}

(bar.c)
int bar(int x) {
  return foo(x);
}
```

At LTO time when `foo.o` is imported into `bar.o`, function `foo` might be inlined into `bar` and PGO-guided indirect call promotion will run after that. If the profile data tells that the promotion of `@f1` or `@f2` is beneficial, the optimizer will check if the "promoted" `@f1` or `@f2` (such as `@f1.llvm.0` or `@f2.llvm.0`) is available. Without this patch, importing `!callees` metadata would only add promoted declarations of `@f1` and `@f2` to the `bar.o`, but still the optimizer will assume that the function is available and perform the promotion. The result of that is link failure with `undefined reference to @f1.llvm.0`.

This patch fixes this problem by adding callees in the `!callees` metadata to CallGraphEdges so that their definition would be properly imported into.

One may ask that there already is a logic to add indirect call promotion targets to be added to CallGraphEdges. However, if profile data says "indirect call promotion is only beneficial under a certain inline context", the logic wouldn't work. In the code example above, if profile data is like
```
bar:1000000:100000
  1:100000
    1: foo:100000
        1: 100000 f1:100000
```
, Computing FunctionSummary for `foo.o` wouldn't add `foo->f1` to CallGraphEdges. (Also, it is at least "possible" that one can provide profile data to only link step but not to compilation step).

Reviewers: tejohnson, mehdi_amini, pcc

Reviewed By: tejohnson

Subscribers: inglorion, eraman, llvm-commits

Differential Revision: https://reviews.llvm.org/D44399

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@327358 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Analysis/ModuleSummaryAnalysis.cpp

@@ -291,6 +291,18 @@ computeFunctionSummary(ModuleSummaryIndex &Index, const Module &M,
         if (!CalledValue || isa<Constant>(CalledValue))
           continue;
 
+        // Check if the instruction has a callees metadata. If so, add callees
+        // to CallGraphEdges to reflect the references from the metadata, and
+        // to enable importing for subsequent indirect call promotion and
+        // inlining.
+        if (auto *MD = I.getMetadata(LLVMContext::MD_callees)) {
+          for (auto &Op : MD->operands()) {
+            Function *Callee = mdconst::extract_or_null<Function>(Op);
+            if (Callee)
+              CallGraphEdges[Index.getOrInsertValueInfo(Callee)];
+          }
+        }
+
         uint32_t NumVals, NumCandidates;
         uint64_t TotalCount;
         auto CandidateProfileData =