[IPO] Add documentation for new function argument

[smeenai]

Summary

As pointed out by @fhahn in https://reviews.llvm.org/D146876.

[tejohnson]

Thanks! lgtm with minor wording tweak suggested

llvm/include/llvm/Transforms/IPO/FunctionImport.h

139

Maybe just "callback" instead of "function-like object"?

[smeenai]

llvm/include/llvm/Transforms/IPO/FunctionImport.h

139

Yup, that's the word I was looking for :D

llvm/include/llvm/Transforms/IPO/FunctionImport.h

public:
   PreservedAnalyses run(Module &M, ModuleAnalysisManager &AM);
 };
 
 /// Compute all the imports and exports for every module in the Index.
 ///
 /// \p ModuleToDefinedGVSummaries contains for each Module a map
 /// (GUID -> Summary) for every global defined in the module.
 ///
+/// \p isPrevailing is a callback that will be called with a global value's GUID
+/// and summary and should return whether the module corresponding to the
+/// summary contains the linker-prevailing copy of that value.
+///
 /// \p ImportLists will be populated with an entry for every Module we are
 /// importing into. This entry is itself a map that can be passed to
 /// FunctionImporter::importFunctions() above (see description there).
 ///
 /// \p ExportLists contains for each Module the set of globals (GUID) that will
 /// be imported by another module, or referenced by such a function. I.e. this
 /// is the set of globals that need to be promoted/renamed appropriately.
 void ComputeCrossModuleImport(
     const ModuleSummaryIndex &Index,
     const StringMap<GVSummaryMapTy> &ModuleToDefinedGVSummaries,
     function_ref<bool(GlobalValue::GUID, const GlobalValueSummary *)>
         isPrevailing,
     StringMap<FunctionImporter::ImportMapTy> &ImportLists,
     StringMap<FunctionImporter::ExportSetTy> &ExportLists);
 
 /// Compute all the imports for the given module using the Index.
 ///
+/// \p isPrevailing is a callback that will be called with a global value's GUID
+/// and summary and should return whether the module corresponding to the
+/// summary contains the linker-prevailing copy of that value.
+///
 /// \p ImportList will be populated with a map that can be passed to
 /// FunctionImporter::importFunctions() above (see description there).
 void ComputeCrossModuleImportForModule(
     StringRef ModulePath,
     function_ref<bool(GlobalValue::GUID, const GlobalValueSummary *)>
         isPrevailing,
     const ModuleSummaryIndex &Index, FunctionImporter::ImportMapTy &ImportList);