This patch replaces LLVM_ATTRIBUTE_UNUSED with [[maybe_unused]],
introduced as part of C++17.

Handles clang::DiagnosticsEngine and clang::DiagnosticIDs.

For DiagnosticIDs, this mostly migrates from `new DiagnosticIDs` to
convenience method `DiagnosticIDs::create()`.

Part of cleanup https://github.com/llvm/llvm-project/issues/151026

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 reverts commit e2a885537f11f8d9ced1c80c2c90069ab5adeb1d. Build failures were fixed right away and reverting the original commit without the fixes breaks the build again.

This reverts commit 9e306ad4600c4d3392c194a8be88919ee758425c.

Multiple builtbot failures have been reported:
https://github.com/llvm/llvm-project/pull/139584

The `DiagnosticOptions` class is currently intrusively
reference-counted, which makes reasoning about its lifetime very
difficult in some cases. For example, `CompilerInvocation` owns the
`DiagnosticOptions` instance (wrapped in `llvm::IntrusiveRefCntPtr`) and
only exposes an accessor returning `DiagnosticOptions &`. One would
think this gives `CompilerInvocation` exclusive ownership of the object,
but that's not the case:

```c++
void shareOwnership(CompilerInvocation &CI) {
  llvm::IntrusiveRefCntPtr<DiagnosticOptions> CoOwner = &CI.getDiagnosticOptions();
  // ...
}
```

This is a perfectly valid pattern that is being actually used in the
codebase.

I would like to ensure the ownership of `DiagnosticOptions` by
`CompilerInvocation` is guaranteed to be exclusive. This can be
leveraged for a copy-on-write optimization later on. This PR changes
usages of `DiagnosticOptions` across `clang`, `clang-tools-extra` and
`lldb` to not be intrusively reference-counted.

Summary:
The logic to strip positional args feels very fragile, but it's terribly
useful
when you want to use a tool on a file and have the exact argv.

Today doesn't work with header-parsing actions because these are
"precompile"
rather than "compile", from tooling's perspective it's all the same.

Reviewers:
kadircet

Subscribers:

This patch replaces uses of StringRef::{starts,ends}with with
StringRef::{starts,ends}_with for consistency with
std::{string,string_view}::{starts,ends}_with in C++20.

I'm planning to deprecate and eventually remove
StringRef::{starts,ends}with.

The forwarding header is left in place because of its use in
`polly/lib/External/isl/interface/extract_interface.cc`, but I have
added a GCC warning about the fact it is deprecated, because it is used
in `isl` from where it is included by Polly.