The overloads accepting a string will be deprecated soon, similar to
other functions in TargetRegistry.

These are identified by misc-include-cleaner.  I've filtered out those
that break builds.  Also, I'm staying away from llvm-config.h,
config.h, and Compiler.h, which likely cause platform- or
compiler-specific build failures.

This avoids doing a Triple -> std::string -> Triple round trip in lots
of places, now that the Module stores a Triple.

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.

This change allows to expose through an interface attributes wrapping
content as external resources, and the usage inside the ModuleToObject
show how we will be able to provide runtime libraries without relying on
the filesystem.

Here is the [merged
MR](https://github.com/llvm/llvm-project/pull/116007) which caused a
failure and [was
reverted](https://github.com/llvm/llvm-project/pull/116811).

Thanks to @joker-eph for the help, I fix it (miss constructing
`ModuleObject` with callback functions in
`mlir/lib/Target/LLVM/NVVM/Target.cpp`) and split unit tests from origin
test which don't need `ptxas` to make the test runs more widely.

Reverts llvm/llvm-project#116007

Bot is broken.

In ModuleToObject flow, users may want to add some callback functions
invoked with LLVM IR/ISA for debugging or other purposes.

Excluded updates to mlir/lib/AsmParser/Parser.cpp ,
which caused LIT failure "FAIL: MLIR::completion.test" on multiple buildbots.

This reverts commit 5a6e52d6ef96d2bcab6dc50bdb369662ff17d2a0.

"FAIL: MLIR::completion.test" on multiple buildbots.