We can use *Set::insert_range to replace "for" loop-based insertions.
In some cases, we can further fold insert_range into the set
declaration.

We can use *Set::insert_range to collapse:

  for (auto Elem : Range)
    Set.insert(E);

down to:

  Set.insert_range(Range);

In some cases, we can further fold that into the set declaration.

The module currently stores the target triple as a string. This means
that any code that wants to actually use the triple first has to
instantiate a Triple, which is somewhat expensive. The change in #121652
caused a moderate compile-time regression due to this. While it would be
easy enough to work around, I think that architecturally, it makes more
sense to store the parsed Triple in the module, so that it can always be
directly queried.

For this change, I've opted not to add any magic conversions between
std::string and Triple for backwards-compatibilty purses, and instead
write out needed Triple()s or str()s explicitly. This is because I think
a decent number of them should be changed to work on Triple as well, to
avoid unnecessary conversions back and forth.

The only interesting part in this patch is that the default triple is
Triple("") instead of Triple() to preserve existing behavior. The former
defaults to using the ELF object format instead of unknown object
format. We should fix that as well.

#125983

Follow up to https://github.com/llvm/llvm-project/issues/123569

`llvm::Type::getPointerTo()` is to be deprecated.
Replace remaining uses of it.

Building with -DLLVM_ENABLE_EXPORTED_SYMBOLS_IN_EXECUTABLES=Off should not
prevent use of bugpoint plugins.

This fix uses the approach implemented in
https://github.com/llvm/llvm-project/pull/101741.