[sil-linker] Improve the implementation of the hasSILFunction API

It it now possible to check if a function with a given name and a given linkage exists in one of the modules,
even if the current module contains a function with this name but a difference linkage.
This is useful e.g. for performing a lookup of pre-specializations.

lib/SIL/Linker.cpp

@@ -139,6 +139,10 @@ SILFunction *SILLinkerVisitor::lookupFunction(StringRef Name,
   return NewFn;
 }
 
+/// Process Decl, recursively deserializing any thing Decl may reference.
+bool SILLinkerVisitor::hasFunction(StringRef Name, SILLinkage Linkage) {
+  return  Loader->hasSILFunction(Name, Linkage);
+}
 
 /// Deserialize the VTable mapped to C if it exists and all SIL the VTable
 /// transitively references.