[clangd] Names that are not spelled in source code are reserved.

Summary:
These are often not expected to be used directly e.g.
```
TEST_F(Fixture, X) {
  ^  // "Fixture_X_Test" expanded in the macro should be down ranked.
}
```

Only doing this for sema for now, as such symbols are mostly coming from sema
e.g. gtest macros expanded in the main file. We could also add a similar field
for the index symbol.

Reviewers: sammccall

Reviewed By: sammccall

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

Differential Revision: https://reviews.llvm.org/D53374

llvm-svn: 344736