[clangd] Better handling symbols defined in macros.

Summary:
For symbols defined inside macros:
 * use expansion location, if the symbol is formed via macro concatenation.
 * use spelling location, otherwise.

This will fix some symbols that have ill-format location (especial invalid filepath).

Reviewers: ioeric

Reviewed By: ioeric

Subscribers: klimek, ilya-biryukov, jkorous-apple, cfe-commits

Differential Revision: https://reviews.llvm.org/D42575

llvm-svn: 323867