[clangd] Introduce a "Symbol" class.

Summary:
* The "Symbol" class represents a C++ symbol in the codebase, containing all the
  information of a C++ symbol needed by clangd. clangd will use it in clangd's
  AST/dynamic index and global/static index (code completion and code
  navigation).
* The SymbolCollector (another IndexAction) will be used to recollect the
  symbols when the source file is changed (for ASTIndex), or to generate
  all C++ symbols for the whole project.

In the long term (when index-while-building is ready), clangd should share a
same "Symbol" structure and IndexAction with index-while-building, but
for now we want to have some stuff working in clangd.

Reviewers: ioeric, sammccall, ilya-biryukov, malaperle

Reviewed By: sammccall

Subscribers: malaperle, klimek, mgorny, cfe-commits

Differential Revision: https://reviews.llvm.org/D40897

llvm-svn: 320486