[clangd] Collect enum constants in SymbolCollector

Summary:
* ignore nameless symbols
* include enum constant declarataion

Reviewers: ilya-biryukov, jkorous-apple

Reviewed By: ilya-biryukov

Subscribers: ioeric, cfe-commits, klimek

Differential Revision: https://reviews.llvm.org/D42074

llvm-svn: 322929