[libc][CPP] make the string trap on OOM (#172260)

This PR makes the string trap on OOM.

Previously, the `__builtin_unreachable` has made debugging tricky as it
makes the control flow of OOM as an undefined behavior.
We can run into OOM with testing configuration easily where memory is
statically bounded.

We did not settle with the best solution of this but making it trap is
at least better than UB
in this case.

libc/src/__support/CPP/string.h

@@ -132,13 +132,16 @@ public:
     // by 8 is cheap. We guard the extension so the operation doesn't overflow.
     if (new_capacity < SIZE_MAX / 11)
       new_capacity = new_capacity * 11 / 8;
+
     if (void *Ptr = ::realloc(buffer_ == get_empty_string() ? nullptr : buffer_,
                               new_capacity)) {
       buffer_ = static_cast<char *>(Ptr);
       capacity_ = new_capacity;
-    } else {
-      __builtin_unreachable(); // out of memory
+      return;
     }
+    // Out of memory: this is not handled in current implementation,
+    // We trap the program and exits.
+    __builtin_trap();
   }
 
   LIBC_INLINE void resize(size_t size) {