[clangd] Implement format on type

Summary:
This patch adds onTypeFormatting to clangd.

The trigger character is '}' and it works by scanning for the matching '{' and formatting the range in-between.

There are problems with ';' as a trigger character, the cursor position is before the `|`:
```
int main() {
  int i;|
}
```
becomes:
```
int main() {  int i;| }
```
which is not likely what the user intended.

Also formatting at semicolon in a non-properly closed scope puts the following tokens in the same unwrapped line, which doesn't reformat nicely.

Reviewers: bkramer

Reviewed By: bkramer

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D29990

llvm-svn: 295304