[sil-module] Create SILFunctionBuilder and hide creation/erasing functions on SILModule.

This commit does not modify those APIs or their usage. It just:

1. Moves the APIs onto SILFunctionBuilder and makes SILFunctionBuilder a friend
   of SILModule.
2. Hides the APIs on SILModule so all users need to use SILFunctionBuilder to
   create/destroy functions.

I am doing this in order to allow for adding/removing function notifications to
be enforced via the type system in the SILOptimizer. In the process of finishing
off CallerAnalysis for FSO, I discovered that we were not doing this everywhere
we need to. After considering various other options such as:

1. Verifying after all passes that the notifications were sent correctly and
   asserting. Turned out to be expensive.
2. Putting a callback in SILModule. This would add an unnecessary virtual call.

I realized that by using a builder we can:

1. Enforce that users of SILFunctionBuilder can only construct composed function
   builders by making the composed function builder's friends of
   SILFunctionBuilder (notice I did not use the word subclass, I am talking
   about a pure composition).
2. Refactor a huge amount of code in SILOpt/SILGen that involve function
   creation onto a SILGenFunctionBuilder/SILOptFunctionBuilder struct. Many of
   the SILFunction creation code in question are straight up copies of each
   other with small variations. A builder would be a great way to simplify that
   code.
3. Reduce the size of SILModule.cpp by 25% from ~30k -> ~23k making the whole
   file easier to read.

NOTE: In this commit, I do not hide the constructor of SILFunctionBuilder since
I have not created the derived builder structs yet. Once I have created those in
a subsequent commit, I will hide that constructor.

rdar://42301529

include/swift/SIL/SILModule.h

@@ -65,6 +65,7 @@ class ModuleDecl;
 class SILUndef;
 class SourceFile;
 class SerializedSILLoader;
+class SILFunctionBuilder;
 
 namespace Lowering {
 class SILGenModule;
@@ -103,6 +104,8 @@ enum class SILStage {
 /// \brief A SIL module. The SIL module owns all of the SILFunctions generated
 /// when a Swift compilation context is lowered to SIL.
 class SILModule {
+  friend class SILFunctionBuilder;
+
 public:
   using FunctionListType = llvm::ilist<SILFunction>;
   using GlobalListType = llvm::ilist<SILGlobalVariable>;
@@ -531,51 +534,6 @@ public:
   /// the same AST module.
   void linkAllFromCurrentModule();
 
-  /// \brief Return the declaration of a utility function that can,
-  /// but needn't, be shared between modules.
-  SILFunction *getOrCreateSharedFunction(SILLocation loc, StringRef name,
-                                         CanSILFunctionType type,
-                                         IsBare_t isBareSILFunction,
-                                         IsTransparent_t isTransparent,
-                                         IsSerialized_t isSerialized,
-                                         ProfileCounter entryCount,
-                                         IsThunk_t isThunk);
-
-  /// \brief Return the declaration of a function, or create it if it doesn't
-  /// exist.
-  SILFunction *getOrCreateFunction(
-      SILLocation loc, StringRef name, SILLinkage linkage,
-      CanSILFunctionType type, IsBare_t isBareSILFunction,
-      IsTransparent_t isTransparent, IsSerialized_t isSerialized,
-      ProfileCounter entryCount = ProfileCounter(),
-      IsThunk_t isThunk = IsNotThunk,
-      SubclassScope subclassScope = SubclassScope::NotApplicable);
-
-  /// \brief Return the declaration of a function, or create it if it doesn't
-  /// exist.
-  SILFunction *
-  getOrCreateFunction(SILLocation loc, SILDeclRef constant,
-                      ForDefinition_t forDefinition,
-                      ProfileCounter entryCount = ProfileCounter());
-
-  /// \brief Create a function declaration.
-  ///
-  /// 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 *
-  createFunction(SILLinkage linkage, StringRef name,
-                 CanSILFunctionType loweredType, GenericEnvironment *genericEnv,
-                 Optional<SILLocation> loc, IsBare_t isBareSILFunction,
-                 IsTransparent_t isTrans, IsSerialized_t isSerialized,
-                 ProfileCounter entryCount = ProfileCounter(),
-                 IsThunk_t isThunk = IsNotThunk,
-                 SubclassScope subclassScope = SubclassScope::NotApplicable,
-                 Inline_t inlineStrategy = InlineDefault,
-                 EffectsKind EK = EffectsKind::Unspecified,
-                 SILFunction *InsertBefore = nullptr,
-                 const SILDebugScope *DebugScope = nullptr);
-
   /// Look up the SILWitnessTable representing the lowering of a protocol
   /// conformance, and collect the substitutions to apply to the referenced
   /// witnesses, if any.