[clangd] Add "member" symbols to the index

Summary:
This adds more symbols to the index:
- member variables and functions
- enum constants in scoped enums

The code completion behavior should remain intact but workspace symbols should
now provide much more useful symbols.
Other symbols should be considered such as the ones in "main files" (files not
being included) but this can be done separately as this introduces its fair
share of problems.

Signed-off-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>

Reviewers: ioeric, sammccall

Reviewed By: ioeric, sammccall

Subscribers: hokein, sammccall, jkorous, klimek, ilya-biryukov, jkorous-apple, ioeric, MaskRay, cfe-commits

Differential Revision: https://reviews.llvm.org/D44954

llvm-svn: 334017