Introduce FindIfOrNull() FindIfOrNone() and Contains() (#5322)

`FindIfOrNull` returns a pointer to the element in the range if it's
found, and nullptr otherwise. `FindIfOrNone` returns a copy of the
element in the range if it's found, and `T::None` (for a range of
elements of type `T`) otherwise. `Contains` returns a bool indicating
whether the element in the range is found.

These functions replace `llvm::find()` and `llvm::find_if()` when you
want a single answer back instead of an iterator. This avoids the need
to check against `end()`, allowing the return condition to be tested as
a standard bool.

We replace uses of `find()` and `find_if()` that did not require an
iterator with these new helpers.

Note that the return type of `FindIfOrNull` is a pointer since we can
not write `optional<T&>`, which must be tested for null. If the null
check is omitted, UB occurs and the resulting code may end up with an
incorrect pointer (https://crbug.com/40153300) into the range (or
elsewhere), rather than a null dereference. And this would be very
confusing to debug. Hopefully debug builds and sanitizers keep this from
being an issue we sink a bunch of time into debugging.

toolchain/check/handle_function.cpp

@@ -5,6 +5,7 @@
 #include <optional>
 #include <utility>
 
+#include "common/find.h"
 #include "toolchain/base/kind_switch.h"
 #include "toolchain/check/context.h"
 #include "toolchain/check/control_flow.h"
@@ -90,15 +91,9 @@ static auto FindSelfPattern(Context& context,
     -> SemIR::InstId {
   auto implicit_param_patterns =
       context.inst_blocks().GetOrEmpty(implicit_param_patterns_id);
-  if (const auto* i = llvm::find_if(implicit_param_patterns,
-                                    [&](auto implicit_param_id) {
-                                      return SemIR::IsSelfPattern(
-                                          context.sem_ir(), implicit_param_id);
-                                    });
-      i != implicit_param_patterns.end()) {
-    return *i;
-  }
-  return SemIR::InstId::None;
+  return FindIfOrNone(implicit_param_patterns, [&](auto implicit_param_id) {
+    return SemIR::IsSelfPattern(context.sem_ir(), implicit_param_id);
+  });
 }
 
 // Diagnoses issues with the modifiers, removing modifiers that shouldn't be