[clangd] Fix a case where we fail to detect a header-declared symbol in rename.

Summary:
Failing case:

```
 #include "foo.h"
 void fo^o() {}
```

getRenameDecl() returns the decl of the symbol under the cursor (which is
in the current main file), instead, we use the canonical decl to determine
whether a symbol is declared in #included header.

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D63872

llvm-svn: 364537