[RFC][C++20][Modules] Fix crash when function and lambda inside loaded from different modules

Summary:
Because AST loading code is lazy and happens in unpredictable order it could happen that function and lambda inside function can be loaded from different modules. In this case, captured DeclRefExpr won’t match the corresponding VarDecl inside function. In AST it looks like this:
```
FunctionDecl 0x555564f4aff0 <Conv.h:33:1, line:41:1> line:33:35 imported in ./thrift_cpp2_base.h hidden tryTo 'Expected<Tgt, const char *> ()' inline
|-also in ./folly-conv.h
`-CompoundStmt 0x555564f7cfc8 <col:43, line:41:1>
  |-DeclStmt 0x555564f7ced8 <line:34:3, col:17>
  | `-VarDecl 0x555564f7cef8 <col:3, col:16> col:7 imported in ./thrift_cpp2_base.h hidden referenced result 'Tgt' cinit
  |   `-IntegerLiteral 0x555564f7d080 <col:16> 'int' 0
  |-CallExpr 0x555564f7cea8 <line:39:3, col:76> '<dependent type>'
  | |-UnresolvedLookupExpr 0x555564f7bea0 <col:3, col:19> '<overloaded function type>' lvalue (no ADL) = 'then_' 0x555564f7bef0
  | |-CXXTemporaryObjectExpr 0x555564f7bcb0 <col:25, col:45> 'Expected<bool, int>':'folly::Expected<bool, int>' 'void () noexcept' zeroing
  | `-LambdaExpr 0x555564f7bc88 <col:48, col:75> '(lambda at Conv.h:39:48)'
  |   |-CXXRecordDecl 0x555564f76b88 <col:48> col:48 imported in ./folly-conv.h hidden implicit <undeserialized declarations> class definition
  |   | |-also in ./thrift_cpp2_base.h
  |   | `-DefinitionData lambda empty standard_layout trivially_copyable literal can_const_default_init
  |   |   |-DefaultConstructor defaulted_is_constexpr
  |   |   |-CopyConstructor simple trivial has_const_param needs_implicit implicit_has_const_param
  |   |   |-MoveConstructor exists simple trivial needs_implicit
  |   |   |-CopyAssignment trivial has_const_param needs_implicit implicit_has_const_param
  |   |   |-MoveAssignment
  |   |   `-Destructor simple irrelevant trivial constexpr needs_implicit
  |   `-CompoundStmt 0x555564f7d1a8 <col:58, col:75>
  |     `-ReturnStmt 0x555564f7d198 <col:60, col:67>
  |       `-DeclRefExpr 0x555564f7d0a0 <col:67> 'Tgt' lvalue Var 0x555564f7d0c8 'result' 'Tgt' refers_to_enclosing_variable_or_capture
  `-ReturnStmt 0x555564f7bc78 <line:40:3, col:11>
    `-InitListExpr 0x555564f7bc38 <col:10, col:11> 'void'
```
I’m not sure that it is the right fix for the problem so any ideas how to fix it better are very appreciated.

Test Plan: check-clang

Author: dmpolukhin

clang/lib/Sema/SemaTemplateInstantiate.cpp

@@ -4387,6 +4387,11 @@ LocalInstantiationScope::findInstantiationOf(const Decl *D) {
       // a previous declaration.
       if (const TagDecl *Tag = dyn_cast<TagDecl>(CheckD))
         CheckD = Tag->getPreviousDecl();
+      else if (const VarDecl *VD = dyn_cast<VarDecl>(CheckD))
+        // Check re-declaration chain for variable to deduplicate variables
+        // that might be captured inside lambdas. Function and lambda class
+        // inside can be loaded from different modules.
+        CheckD = VD->getPreviousDecl();
       else
         CheckD = nullptr;
     } while (CheckD);

clang/lib/Serialization/ASTReaderDecl.cpp

@@ -3291,6 +3291,13 @@ DeclContext *ASTDeclReader::getPrimaryContextForMerging(ASTReader &Reader,
   if (auto *TU = dyn_cast<TranslationUnitDecl>(DC))
     return TU->getPrimaryContext();
 
+  // Merge VarDecls inside functions to deduplicate variables that might be
+  // captured inside lambdas. Function and lambda class inside can be loaded
+  // from different modules.
+  if (auto *FD = dyn_cast<FunctionDecl>(DC))
+    if (FD->getOwningModule())
+      return FD->getCanonicalDecl();
+
   return nullptr;
 }