Make SILFunction::create() private and change all direct uses of SILFunction::create() to instead use SILMod.getOrCreateFunction().

This centralizes the entrypoints for creating SILFunctions. Creating a
SILFunction is intimately tied to a specific SILModule, so it makes sense to
either centralize the creation on SILModule or SILFunction. Since a SILFunction
is in a SILModule, it seems more natural to put it on SILModule.

I purposely created a new override on SILMod that exactly matches the signature
of SILFunction::create so that beyond the extra indirection through SILMod, this
change should be NFC. We can refactor individual cases in later iterations of
refactoring.

include/swift/SIL/SILModule.h

@@ -443,6 +443,27 @@ public:
                                    SILDeclRef constant,
                                    ForDefinition_t forDefinition);
 
+  /// \brief Return the declaration of a function, or create it if it does not
+  /// exist.
+  ///
+  /// This signature is a direct copy of the signature of SILFunction::create()
+  /// in order to simplify refactoring all SILFunction creation use-sites to use
+  /// SILModule. Eventually the uses should probably be refactored.
+  SILFunction *getOrCreateFunction(SILLinkage linkage, StringRef name,
+                                   CanSILFunctionType loweredType,
+                                   GenericParamList *contextGenericParams,
+                                   Optional<SILLocation> loc,
+                                   IsBare_t isBareSILFunction,
+                                   IsTransparent_t isTrans,
+                                   IsFragile_t isFragile,
+                                   IsThunk_t isThunk = IsNotThunk,
+                                   SILFunction::ClassVisibility_t classVisibility = SILFunction::NotRelevant,
+                                   Inline_t inlineStrategy = InlineDefault,
+                                   EffectsKind EK = EffectsKind::Unspecified,
+                                   SILFunction *InsertBefore = nullptr,
+                                   const SILDebugScope *DebugScope = nullptr,
+                                   DeclContext *DC = nullptr);
+
   /// Look up the SILWitnessTable representing the lowering of a protocol
   /// conformance, and collect the substitutions to apply to the referenced
   /// witnesses, if any.