[clangd] Add main file macros into the main-file index.

This patch is a try to fix `WorkspaceSymbols.Macros` test after D93796.
If a macro definition is in the preamble section, then it appears to be in the preamble (static) index and not in the main-file (dynamic) index.
Thus, a such macro could not be found at a symbol search according to the logic that we skip symbols from the static index if the location of these symbols is inside the dynamic index files.
To fix this behavior this patch adds main file macros into the main-file (dynamic) index.

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D94477