Remove SourceFile::addVisibleDecl.

`SourceFile::addVisibleDecl` is an unnecessary API.
It was upstreamed in swiftlang#30821.

Author: dan-zheng

include/swift/AST/SourceFile.h

@@ -394,8 +394,6 @@ class SourceFile final : public FileUnit {
   void cacheVisibleDecls(SmallVectorImpl<ValueDecl *> &&globals) const;
   const SmallVectorImpl<ValueDecl *> &getCachedVisibleDecls() const;
 
-  void addVisibleDecl(ValueDecl *decl);
-
   virtual void lookupValue(DeclName name, NLKind lookupKind,
                            SmallVectorImpl<ValueDecl*> &result) const override;
 

lib/AST/Module.cpp

@@ -2232,11 +2232,6 @@ SourceFile::getCachedVisibleDecls() const {
   return getCache().AllVisibleValues;
 }
 
-void SourceFile::addVisibleDecl(ValueDecl *decl) {
-  Decls->push_back(decl);
-  getCache().AllVisibleValues.push_back(decl);
-}
-
 static void performAutoImport(
     SourceFile &SF,
     SourceFile::ImplicitModuleImportKind implicitModuleImportKind) {

lib/SILOptimizer/Utils/Differentiation/LinearMapInfo.cpp

@@ -156,7 +156,7 @@ LinearMapInfo::createBranchingTraceDecl(SILBasicBlock *originalBB,
   computeAccessLevel(branchingTraceDecl, original->getEffectiveSymbolLinkage());
   branchingTraceDecl->getInterfaceType();
   assert(branchingTraceDecl->hasInterfaceType());
-  file.addVisibleDecl(branchingTraceDecl);
+  file.addTopLevelDecl(branchingTraceDecl);
   // Add basic block enum cases.
   for (auto *predBB : originalBB->getPredecessorBlocks()) {
     auto bbId = "bb" + std::to_string(predBB->getDebugID());
@@ -226,7 +226,7 @@ LinearMapInfo::createLinearMapStruct(SILBasicBlock *originalBB,
   computeAccessLevel(linearMapStruct, original->getEffectiveSymbolLinkage());
   linearMapStruct->getInterfaceType();
   assert(linearMapStruct->hasInterfaceType());
-  file.addVisibleDecl(linearMapStruct);
+  file.addTopLevelDecl(linearMapStruct);
   return linearMapStruct;
 }