[clangd] Collect references for externally visible main-file symbols

Summary:
Without this patch clangd does not collect references for main-file symbols if there is no public declaration in preamble.
Example:
`test1.c`
```
void f1() {}
```

`test2.c`
```
extern void f1();
void f2() {
  f^1();
}
```
`Find all references` does not show definition of f1() in the result, but GTD works OK.

Reviewers: sammccall, kadircet

Reviewed By: kadircet

Subscribers: ilya-golovenko, ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D84513