[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

GitOrigin-RevId: dea74b2820612ecca967528f7f2bbcc6d05801fe
diff --git a/include/__string b/include/__string
index d8b672e..5b340d0 100644
--- a/include/__string
+++ b/include/__string
@@ -953,7 +953,7 @@
 template <class _CharT, class _Traits>
 inline _LIBCPP_CONSTEXPR_AFTER_CXX11 const _CharT *
 __search_substring(const _CharT *__first1, const _CharT *__last1,
-                   const _CharT *__first2, const _CharT *__last2) {
+                   const _CharT *__first2, const _CharT *__last2) _NOEXCEPT {
   // Take advantage of knowing source and pattern lengths.
   // Stop short when source is smaller than pattern.
   const ptrdiff_t __len2 = __last2 - __first2;