[clang][AST] Fix Wreturn-type gcc warning (NFC)

GCC warning:
```
/llvm-project/clang-tools-extra/clangd/SemanticHighlighting.cpp: In function ‘bool clang::clangd::{anonymous}::canHighlightName(clang::DeclarationName)’:
/llvm-project/clang-tools-extra/clangd/SemanticHighlighting.cpp:64:1: warning: control reaches end of non-void function [-Wreturn-type]
   64 | }
      | ^
```