[ClangImporter] Avoid use-after-free of clang::DiagnosticOptions after rebranch (#85445)
Upstream LLVM in llvm/llvm-project#139584 changed `DiagnosticOptions`
from being a referenced counted object to just be a reference, not owned
by the `clang::DiagnosticEngine`.
In 0981b71090622100019bcd857a62262e2f96bda9 (part of #82243), the usages
of the Swift repository were adapted to the new memory model, but it
introduced at least one use-after-free and a potential one around the
usage of Clang in the Clang Importer.
This commit tries to fix the use-after-free in both cases, by returning
a `unique_ptr` to the `clang::DiagnosticOptions`, which makes the
lifetime of the `DiagnosticOptions` match the lifetime of the variable
that uses it (normally a `CompilerInvocation`).
Other cases in 0981b71090622100019bcd857a62262e2f96bda9 should be safe
because the lifetime of the `DiagnosticOptions` do not seem to propagate
beyond the scope of the functions where they live (but I am not fully
sure about the one in `IDETool/CompilerInvocation.cpp` completely).
This was causing compiler crashes during the test
`Interop/Cxx/stdlib/unsupported-stdlib.swift` which eventually uses
`createClangDriver` and tries to emit a diagnostic, which in some cases
was reading the memory from `DiagnosticOptions` when it was already out
of scope.
swift
9eca612b - [ClangImporter] Avoid use-after-free of clang::DiagnosticOptions after rebranch (#85445)
