[clangd] Extend CollectMainFileMacros.

Extend the existing MainFileMacros structure:
- record more information (InConditionalDirective) in MacroOccurrence
- collect macro references inside macro body (fix a long-time FIXME)

So that the MainFileMacros preseve enough information, which allows a
just-in-time convertion to interop with include-cleaner::Macro for
include-cleaer features.

See the context in https://reviews.llvm.org/D146017.

Differential Revision: https://reviews.llvm.org/D146279