[clang] Introduce paged vector (PR #66430)

[Vassil Vassilev via cfe-commits]

================
@@ -1625,6 +1625,38 @@ SmallVector has grown a few other minor advantages over std::vector, causing
    and is no longer "private to the implementation". A name like
    ``SmallVectorHeader`` might be more appropriate.
 
+.. _dss_pagedvector:
+
+llvm/ADT/PagedVector.h
+^^^^^^^^^^^^^^^^^^^^^^
+
+``PagedVector<Type, PageSize>`` is a random access container that allocates
+(PageSize) elements of type Type when the first element of a page is accessed
+via the ``operator[]``.  This is useful for the case in which the number of
+elements is known in advance and their actual initialization is expensive and
+sparse so that it's only done lazily when the element is accessed. When the
+number of used pages is small significant memory savings can be achieved.
+
+The main advantage is that a ``PagedVector`` allows to delay the actual allocation
+of the page until it's needed, at the extra cost of one integer per page and one
+extra indirection when accessing elements with their positional index. 
+
+In order to maximise the memory footprint of this container, it's important to
+balance the PageSize so that it's not too small (otherwise the overhead of the
+integer per page might become too high) and not too big (otherwise the memory is
+wasted if the page is not fully used).
+
+Moreover, while retaining the oder of the elements based on their insertion
----------------
vgvassilev wrote:

That seems unresolved.

https://github.com/llvm/llvm-project/pull/66430