Add APIs to check if a function with a given name exists and to invalidate a SIL linker entry for a function.

These APIs are useful e.g. for quickly finding pre-specialisations by their names.
The existence check is very light-weight and does not try to deserialize bodies of SIL functions.

lib/SIL/SILModule.cpp

@@ -468,6 +468,15 @@ bool SILModule::linkFunction(StringRef Name, SILModule::LinkingMode Mode) {
   return SILLinkerVisitor(*this, getSILLoader(), Mode).processFunction(Name);
 }
 
+SILFunction *SILModule::hasFunction(StringRef Name, SILLinkage Linkage) {
+  assert(!lookUpFunction(Name) && "hasFunction should be only called for "
+                                  "functions that are not contained in the "
+                                  "SILModule yet");
+  return SILLinkerVisitor(*this, getSILLoader(),
+                          SILModule::LinkingMode::LinkNormal)
+      .lookupFunction(Name, Linkage);
+}
+
 void SILModule::linkAllWitnessTables() {
   getSILLoader()->getAllWitnessTables();
 }
@@ -516,6 +525,10 @@ void SILModule::eraseFunction(SILFunction *F) {
   }
 }
 
+void SILModule::invalidateFunctionInSILCache(SILFunction *F) {
+  getSILLoader()->invalidateFunction(F);
+}
+
 /// Erase a global SIL variable from the module.
 void SILModule::eraseGlobalVariable(SILGlobalVariable *G) {
   GlobalVariableMap.erase(G->getName());