[libcxx][test] Make string.modifiers/clear_and_shrink_db1.pass.cpp a regular mode test

Turn this test into a normal mode as it contains well-formed code and
checks for defined behavior. It still can be run in debug mode as of D100866.

Differential Revision: https://reviews.llvm.org/D102192

libcxx/test/libcxx/strings/basic.string/string.modifiers/clear_and_shrink.pass.cpp

@@ -10,11 +10,6 @@
 
 // Call __clear_and_shrink() and ensure string invariants hold
 
-// UNSUPPORTED: libcxx-no-debug-mode
-
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=1
-#define _LIBCPP_ASSERT(x, m) ((x) ? (void)0 : std::exit(0))
-
 #include <string>
 #include <cassert>
 
@@ -39,4 +34,6 @@ int main(int, char**)
     assert(l.size() == 0);
     assert(l.capacity() < cap);
     }
+
+    return 0;
 }