The function currently checks for the command line argument only to
check if compiling for kernel. This is incorrect as the setting can also
be passed programatically.

After pointer element types were removed this function can only return
a GlobalVariable, so reflect that in the type and comments and clean
up callers.

Reviewers: nikic

Reviewed By: nikic

Pull Request: https://github.com/llvm/llvm-project/pull/141323

This used to work because the BTI attribute was taken from the module in
the CodeGen.

e15d67cfc2e5775cc79281aa860f3ad3be628f39 changed that to actually look
at the function attributes. This led to crashes for BTI, because we did
not emit the proper landing pads for the thunk.

Add a new `CreateIntrinsic` overload with no `Types`, useful for
creating calls to non-overloaded intrinsics that don't need additional
mangling.

If left as-is, subsequent optimizations might utilize the possible
memory effects and optimize-out the instrumentation. Think of the
following case:
```
  store i8 4, ptr %shadow
  call void @llvm.lifetime.start.p0(i64 4, ptr %local)
  %28 = call void @foo(ptr %local)
  store i8 -8, ptr %shadow
  call void @llvm.lifetime.end.p0(i64 4, ptr %local)
```

where `foo` is an external function with `memory(argmem: write)`. A pass
such as DeadStoreElimination is allowed to remove the initial store,
which might fail sanitizer checks within `foo`.

My first attempt was to add a `memory(readwrite)` at the call-site
level, but unfortunately the current implementation of
`getMemoryEffects` doesn't exactly give it "precedence" as specified,
but rather restricts the access specified by the call-site and not the
other way around as well.

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.

The meaning of ClRandomSkipRate was inverted in
https://github.com/llvm/llvm-project/pull/88070 but the variable name
was not changed. This patch fixes it to avoid confusion.

Additionally, it elaborates the flag description to mention the
interaction between the random keep rate and hotness cutoff.

If the pointer to be checked is statically known to be zero, the tag
check will always pass since:
1) the tag is zero
2) shadow memory for address 0 is initialized to 0 and never updated.
We can therefore elide the tag check.

We perform the elision in two places:
1) the HWASan pass
2) when lowering the CHECK_MEMACCESS intrinsic. Conceivably, the HWASan
pass may encounter a "cannot currently statically prove to be null"
pointer (and is therefore unable to omit the intrinsic) that later
optimization passes convert into a statically known-null pointer. As a
last line of defense, we perform elision here too.

This also updates the tests from
https://github.com/llvm/llvm-project/pull/122186