[clangd] Implementation of auto type expansion.

Add a tweak for clangd to replace an auto keyword to the deduced type.

This way a user can declare something with auto and then have the
IDE/clangd replace auto with whatever type clangd thinks it is. In case
of long/complext types this makes is reduces writing effort for the
user.

The functionality is similar to the hover over the auto keyword.

Example (from the header):

```
/// Before:
///    auto x = Something();
///    ^^^^
/// After:
///    MyClass x = Something();
///    ^^^^^^^
```

Patch by kuhnel! (Christian Kühnel)
Differential Revision: https://reviews.llvm.org/D62855

llvm-svn: 365792