[clang] [clang-tools-extra] [llvm] [clangd] introduce doxygen parser (PR #150790)

[via llvm-commits]

mccakit wrote:

Yeah, that changed the output though I'm still seeing this

```
/**
 * @brief Adds two numbers
 * @param a First number
 * @param b Second number
 * @return The sum
 */
int add(int a, int b);
 
->
```
/**
  function add
  → int
  Parameters:
  int a - First number
  int b - Second number
  @brief Adds two numbers
  @return The sum
  int add(int a, int b)
```

May I ask you to update the third patch?
I tried merging it myself but there were too many conflicts.
I really want to merge and build it right now

https://github.com/llvm/llvm-project/pull/150790