For lambdas that are converted to C function pointers,
```
int (*ret_zero)() = []() { return 0; };
```
clang will generate conversion method like:
```
CXXConversionDecl implicit used constexpr operator int (*)() 'int (*() const noexcept)()' inline
-CompoundStmt
-ReturnStmt
-ImplicitCastExpr 'int (*)()' <FunctionToPointerDecay>
-DeclRefExpr 'int ()' lvalue CXXMethod 0x5ddb6fe35b18 '__invoke' 'int ()'
-CXXMethodDecl implicit used __invoke 'int ()' static inline
-CompoundStmt (empty)
```
Based on comment in Sema, `__invoke`'s function body is left empty
because it's will be filled in CodeGen, so in AST analysis phase we
should get lambda's `operator()` directly instead of calling `__invoke`
itself.
