[clang] [analyzer] Allow recursive functions to be trivial. (PR #91876)

[Mital Ashok via cfe-commits]

MitalAshok wrote:

You should add a test for mutually recursive functions. I suspect something like this doesn't work:

```c++
    int non_trivial();
    int f(bool b) { return g(!b) + non_trivial(); }
    int g(bool b) { return b ? f(b) : 1; }
    
getFieldTrivial().f(true);  // expected-warning {{...}}
getFieldTrivial().g(true);  // expected-warning {{...}}
```

Since when analyzing `f`, `f` enters the cache as trivial, `g` is analyzed and sees `f` in the cache as trivial, so `g` is marked trivial, but then `f` is marked as non-trivial (so `g` should have been marked as non-trivial, but is marked trivial).

If that is an issue, one way to do this "properly" would be to create a graph, and a function is non-trivial if it has a path to a non-trivial function (this might be too expensive to implement directly). Or we could special case simple recursion only, by changing `TrivialFunctionAnalysisVisitor` to not call `isTrivialImpl` for the current function only.

https://github.com/llvm/llvm-project/pull/91876