Largely, this is just being more specific about where tags get searched
for to remove external dependencies like src/llvm, which reduces the
number of tags *enormously* and significantly increases the usefulness
of the tags file as it is then focusing on 240K lines of Rust code
and 4.5K of C++ rather than just shy of 1M lines of C++ code (mostly
from LLVM) and another 100K lines of Rust tests and a diverse collection
of other languages.

src/rustllvm/RustWrapper.cpp and src/rustllvm/PassWrapper.cpp are
getting tags made, but I'm not sure if that's desirable or not. At
worst, it's not a significant wrong.

A future, desirable step is producing tags for just libstd and libextra
for the use of people using Rust-the-language rather than working on
Rust itself.

By assuming that a recursive type is normalizable within the deeper
calls to `is_normalizable_helper()`, more cases can be handled by this
function.

In order to fix stack overflows, a recursion limit has also been added
for recursive generic type instantiations.

Fix rust-lang#9798
Fix rust-lang#10508
Fix rust-lang#11915

changelog: [`large_enum_variant`]: more precise detection of variants
with large size differences