[generic] initial support for BSC generic function and struct&union · Pull Request !26 · BiShengCLanguage/llvm-project

Now we can parse the following BSC generics(func, struct, union):
```c
#include <stdio.h>
struct S<T>{
  T a;
};
union U<T1, T2>{
  T1 a;
  T2 b;
};
T max<T>(T a, T b) {
  return a > b ? a : b;
}
int main() {
  struct S<int> s1 = {.a = 42}; // ClassTemplateSpecializationDecl
  int a = s1.a;
  printf("Generic Struct Works!\n");
  printf("a is %d\n", a);
  union U<int, float> u1 = {.b = 3.14};
  printf("u1.b is %f\n", u1.b);
  int b = max<int>(2023, 2022);
  printf("Generic Function Works!\n");
  printf("b is %d\n", b);
  return 0;
}
```
Goal: 
- Try to reuse CPP template mechanism inside Clang and make minimun changes to the AST(for the conviniency of source-to-souce).

What we have done:
- Rewrite parsing for `template-parameter-list` like `<T1, T2,...>`, in the end we construct the same data structure inside clang called `TemplateTypeParamDecl` origionally meant for CPP template.
- For BSC generic function, we create AST `FunctionTemplateDecl`; and for BSC generic struct&union, we create AST `ClassTemplateDecl`. For those ASTs, we both have two parts: (1) `TemplateTypeParamDecl` (2) Just normal AST like `FunctionDecl` or `RecordDecl`.
- The instantiation part of generics is still kind of black box to us. The entry point is `Sema::InstantiateClass`, particularlly a visitor will traverse the body of generic type and try to instantiate each Stmt.
```cpp
bool Sema::InstantiateClasss(...){
  ... // before: Member is `T a;`
  Decl* NewMember = Instantiator.Visit(Member);
  ... // after: NewMember is `int a;`
} 
```

TODO:
- Add more test cases to cover the spec and fix bugs when encountered.
- Refactor the code. The demo version is very hacky.
- Add source-to-source support.

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