[clangd] check for synthesized symbols when tracking include locations

This fixes https://github.com/llvm/llvm-project/issues/75115

In C mode with MSVC compatibility, when the `assert` macro is defined,
as a workaround, `static_assert` is implicitly defined as well,
if not already so, in order to work around a broken `assert.h`
implementation.
This workaround was implemented in https://github.com/llvm/llvm-project/commit/8da090381d567d0ec555840f6b2a651d2997e4b3

A synthesized symbol does not occur in source code, and therefore
should not have valid source location, but this was not checked when
inserting this into a `symbol -> include file` map.

The invalid FileID value is used for empty key representation in
the include file hash table, so it's not valid to insert it.