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/parse/extract.cpp

@@ -9,6 +9,7 @@
 #include <utility>
 
 #include "common/error.h"
+#include "common/find.h"
 #include "common/struct_reflection.h"
 #include "toolchain/parse/tree.h"
 #include "toolchain/parse/tree_and_subtrees.h"
@@ -210,7 +211,7 @@ auto NodeExtractor::MatchesNodeIdOneOf(
       *trace_ << "\n";
     }
     return false;
-  } else if (llvm::find(kinds, node_kind) == kinds.end()) {
+  } else if (!Contains(kinds, node_kind)) {
     if (trace_) {
       *trace_ << "NodeIdOneOf error: wrong kind " << node_kind << ", expected ";
       trace_kinds();