Revert "Revert "[ADT] Fix SmallVector unused template instantiation on 32-bit systems.""

This reverts commit ac2635e4.

Revert was too hasty, build was already broken.

llvm/lib/Support/SmallVector.cpp

@@ -78,4 +78,18 @@ void SmallVectorBase<Size_T>::grow_pod(void *FirstEl, size_t MinCapacity,
 }
 
 template class llvm::SmallVectorBase<uint32_t>;
+
+// Disable the uint64_t instantiation for 32-bit builds.
+// Both uint32_t and uint64_t instantations are needed for 64-bit builds.
+// This instantiation will never be used in 32-bit builds, and will cause
+// warnings when sizeof(Size_T) > sizeof(size_t).
+#if SIZE_MAX > UINT32_MAX
 template class llvm::SmallVectorBase<uint64_t>;
+
+// Assertions to ensure this #if stays in sync with SmallVectorSizeType.
+static_assert(sizeof(SmallVectorSizeType<char>) == sizeof(uint64_t),
+              "Expected SmallVectorBase<uint64_t> variant to be in use.");
+#else
+static_assert(sizeof(SmallVectorSizeType<char>) == sizeof(uint32_t),
+              "Expected SmallVectorBase<uint32_t> variant to be in use.");
+#endif