[clangd] Only report explicitly typed symbols during code navigation

Summary:
Clangd was reporting implicit symbols, like results of implicit cast
expressions during code navigation, which is not desired. For example:

```
struct Foo{ Foo(int); };
void bar(Foo);
vod foo() {
  int x;
  bar(^x);
}
```
Performing a GoTo on the point specified by ^ would give two results one
pointing to line `int x` and the other for definition of `Foo(int);`

Reviewers: ilya-biryukov, sammccall

Subscribers: ioeric, MaskRay, jkorous, mgrang, arphaman, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D58495

llvm-svn: 354585