[C++20][Modules] Fix crash when function and lambda inside loaded from different modules (#109167)
Summary:
Because AST loading code is lazy and happens in unpredictable order, it
is possible that a function and lambda inside the function can be loaded
from different modules. As a result, the captured DeclRefExpr won’t
match the corresponding VarDecl inside the function. This situation is
reflected in the AST as follows:
```
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'
```
This diff modifies the AST deserialization process to load lambdas
within the canonical function declaration sooner, immediately following
the function, ensuring that they are loaded from the same module.
Re-land https://github.com/llvm/llvm-project/pull/104512 Added test case
that caused crash due to multiple enclosed lambdas deserialization.
Test Plan: check-clang
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