[clang] Thread Safety Analysis: Basic capability alias-analysis (PR #142955)

[Marco Elver via cfe-commits]

melver wrote:

Just FYI - I rebased the kernel patches, and attempted to apply -Wthread-safety to kernel/sched/, which previously was impossible.

With this PR, it *does* work with modest changes (most are annotations, only few some small code changes): https://git.kernel.org/pub/scm/linux/kernel/git/melver/linux.git/log/?h=cap-analysis/dev

I also discovered that with this form of alias analysis, we can take care of a function acquiring a capability inside a returned object with a hack like this:
```
+#define __acquire_ret(call, ret_expr)		\
+	({					\
+		__auto_type __ret = call;	\
+		__acquire(ret_expr);		\
+		__ret;				\
+	})
+#define __acquires_ret		__no_capability_analysis
...
+ *   struct foo { spinlock_t lock; ... };
+ *   ...
+ *   #define myfunc(...) __acquire_ret(_myfunc(__VA_ARGS__), &__ret->lock)
+ *   struct foo *_myfunc(int bar) __acquires_ret;
+ *   ...
```
... which was necessary for taking care of kernel/sched. 

https://github.com/llvm/llvm-project/pull/142955