[clangd] Implement end-definition-comment inlay hints

This patch implements a new inlay hint feature proposed in https://github.com/clangd/clangd/issues/1634. It introduces a new inlay hint kind BlockEnd which shows a comment-like hint after a definition brace pair, including function/type/namespace. For example,
```
void foo() {
} ^
```
In the code shown above, a hint should be displayed at ^ labelling `// foo`. Such hint only shows when there's no trailing character after the position except whitespaces and optionally ';'.

Also, new configurations are introduced in the inlay hints block
```
InlayHints:
    BlockEnd: Yes # toggling the feature
```

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D150635