[clang] [Clang][Sema] Fix lambdas mistakenly inheriting InImmediateFunctionContext (PR #232458)

[Ferdinand Bachmann via cfe-commits]

Ferdi265 wrote:

@efriedma-quic Just adding the lambda conversion operator to `FunctionDecl::isImmediateEscalating()` ([commit](https://github.com/Ferdi265/llvm-project/commit/afcbed627652b335030654358baa00f6d4c66aaf) / [branch](https://github.com/Ferdi265/llvm-project/tree/fix-defect-lambda-conversion-immediate-escalation)) seems to fix your example, but breaks example `ns1_1` from `clang/test/SemaCXX/cxx23-static-callop-lambda-expression.cpp`

```c++
auto lstatic = []() static consteval  //expected-error{{cannot take address of consteval call}} \
                                      expected-note {{declared here}}
{ return 3; };

// FIXME: the above error should indicate that it was triggered below.
int (*f2)(void) = lstatic;
```

looking at the AST, it looks like the conversion function correctly picked up the `immediate-escalating` tag on the `DeclRefExpr` to the call operator, but the call operator itself was not marked `immediate`, causing the missed error at the call site and making the code compile even though it shouldn't.

```
| |-CXXConversionDecl 0x55e4ab48c6b8 <line:1:16> col:16 implicit used constexpr operator int (*)() 'int (*() const noexcept)()' inline internal-linkage
| | `-CompoundStmt 0x55e4ab4aa1b0 <col:16>
| |   `-ReturnStmt 0x55e4ab4aa1a0 <col:16>
| |     `-ImplicitCastExpr 0x55e4ab4aa188 <col:16> 'int (*)()' <FunctionToPointerDecay>
| |       `-DeclRefExpr 0x55e4ab4aa168 <col:16> 'int ()' lvalue CXXMethod 0x55e4ab48c210 'operator()' 'int ()' immediate-escalating
```

This sounds like another bug related to immediate-escalation of functions/methods, I'll look into it.

https://github.com/llvm/llvm-project/pull/232458