Since we switched from autotools to Meson in commit
94592c14, we don't need to include
`config.h` early to properly enable large file support.  Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.

This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.

Build a table of pre-existing tag types before adding new items.  The
old way would check HasType() each time, which would return true after
the first instance of that tag type had been added, preventing
duplicate tag types to be merged.

This broke duplicate tag types loaded from the state file, because
this code path uses TagBuilder::Complement().

New in clang 3.6.

For callers that already have a Tag instance.

Obsoletes Tag::BeginAdd() and the complicated "bulk add" code.