[clangd] Prefer the definition of a TagDecl (e.g. class) as CanonicalDeclaration.

Summary:
Currently, we pick the first declaration of a symbol in a TU, which is considered
canonical in the clangIndex, as the canonical declaration in clangd. This causes
forward declarations that might appear in a random header to be used as a
canonical declaration, which is not desirable for features like go-to-declaration
or include insertion.

For example, for class X, we would consider the forward declaration in fwd.h to
be the canonical declaration, while the preferred canonical declaration should
be the actual definition in x.h.
```
// fwd.h
class X;  // forward decl

// x.h
class X {};
```

This patch fixes the issue by making symbol collector favor the actual definition of
a TagDecl (i.e. class/struct/enum/union) found in a header file over the first seen
declarations in a TU. Other symbol types like functions are not handled because
using the first seen declarations as canonical declarations is usually a good
heuristic for them.

Reviewers: sammccall

Subscribers: klimek, ilya-biryukov, jkorous-apple, cfe-commits

Differential Revision: https://reviews.llvm.org/D43823

llvm-svn: 326313