[libc++] Add noexcept to string::find and similar members.

Adds `noexcept` to `string_view`/`string::find` and similar members
(`rfind`, etc.). See discussion in D95251. Refs D95821.

Reviewed By: curdeius, ldionne

Differential Revision: https://reviews.llvm.org/D95848

libcxx/test/std/strings/basic.string/string.ops/string_compare/string.pass.cpp

@@ -29,6 +29,7 @@ template <class S>
 void
 test(const S& s, const S& str, int x)
 {
+    LIBCPP_ASSERT_NOEXCEPT(s.compare(str));
     assert(sign(s.compare(str)) == sign(x));
 }