ICU-21174 Add error-checking methods to MaybeStackVector & MemoryPool.

- MaybeStackVector::emplaceBackAndCheckErrorCode()
- MemoryPool::createAndCheckErrorCode()

Started with cherry-picks of:
3b505977c0e82659896125698389b59eabe50b14
63b93bde5c4c9fe030d490d5d448087aa0d4e5fd

icu4c/source/common/cmemory.h

diff --git a/icu4c/source/common/cmemory.h b/icu4c/source/common/cmemory.h
index bdf6ba2..313180b 100644
--- a/icu4c/source/common/cmemory.h
+++ b/icu4c/source/common/cmemory.h
@@ -731,6 +731,18 @@ class MemoryPool : public UMemory {
         return fPool[fCount++] = new T(std::forward<Args>(args)...);
     }
 
+    template <typename... Args>
+    T* createAndCheckErrorCode(UErrorCode &status, Args &&... args) {
+        if (U_FAILURE(status)) {
+            return nullptr;
+        }
+        T *pointer = this->create(args...);
+        if (U_SUCCESS(status) && pointer == nullptr) {
+            status = U_MEMORY_ALLOCATION_ERROR;
+        }
+        return pointer;
+    }
+
     /**
      * @return Number of elements that have been allocated.
      */
@@ -774,6 +786,11 @@ class MaybeStackVector : protected MemoryPool<T, stackCapacity> {
         return this->create(args...);
     }
 
+    template <typename... Args>
+    T *emplaceBackAndCheckErrorCode(UErrorCode &status, Args &&... args) {
+        return this->createAndCheckErrorCode(status, args...);
+    }
+
     int32_t length() const {
         return this->fCount;
     }