Data flow: Fix bad join-order when getAReadContent has large fan-in

Before (terminated before completion)
```
Evaluated relational algebra for predicate DataFlowImplForHttpClientLibraries#c536b619::store#5#fffff@e5ef07bh with tuple counts:
            151500     ~0%    {4} r1 = SCAN DataFlowImplCommon#4f8df883::Cached::store#4#ffff OUTPUT In.1, In.0, In.2, In.3
            150500     ~0%    {5} r2 = JOIN r1 WITH DataFlowImplCommon#4f8df883::Cached::MkTypedContent#fff_20#join_rhs ON FIRST 1 OUTPUT Lhs.1, Lhs.0, Lhs.2, Lhs.3, Rhs.1
            149500     ~0%    {5} r3 = JOIN r2 WITH num#DataFlowImplForHttpClientLibraries#c536b619::TNodeNormal#ff ON FIRST 1 OUTPUT Lhs.2, Lhs.1, Lhs.3, Lhs.4, Rhs.1
            148500     ~0%    {5} r4 = JOIN r3 WITH num#DataFlowImplForHttpClientLibraries#c536b619::TNodeNormal#ff ON FIRST 1 OUTPUT Lhs.3, Lhs.1, Lhs.2, Lhs.4, Rhs.1
        2003849000     ~0%    {5} r5 = JOIN r4 WITH DataFlowPublic#e1781e31::ContentSet::getAReadContent#0#dispred#ff_10#join_rhs ON FIRST 1 OUTPUT Rhs.1, Lhs.1, Lhs.2, Lhs.3, Lhs.4
         105066500  ~9036%    {5} r6 = JOIN r5 WITH project#DataFlowImplForHttpClientLibraries#c536b619::readSet#4#ffff ON FIRST 1 OUTPUT Lhs.3, Lhs.1, Lhs.4, Lhs.2, Rhs.1
                              return r6
```

After
```
Evaluated relational algebra for predicate DataFlowImplForHttpClientLibraries#c536b619::readProj#2#ff@302620cn with tuple counts:
        1461867  ~0%    {2} r1 = SCAN DataFlowPrivate#462ff392::Cached::TContent#f OUTPUT In.0, In.0
        3549054  ~1%    {2} r2 = JOIN r1 WITH DataFlowPublic#e1781e31::ContentSet::getAReadContent#0#dispred#ff_10#join_rhs ON FIRST 1 OUTPUT Rhs.1, Lhs.1
        5772824  ~5%    {2} r3 = JOIN r2 WITH project#DataFlowImplForHttpClientLibraries#c536b619::readSet#4#ffff ON FIRST 1 OUTPUT Lhs.1, Rhs.1
                        return r3

Evaluated relational algebra for predicate DataFlowImplForHttpClientLibraries#c536b619::store#5#fffff@016cd9o1 with tuple counts:
         267905  ~0%    {4} r1 = SCAN DataFlowImplCommon#4f8df883::Cached::store#4#ffff OUTPUT In.1, In.0, In.2, In.3
         267905  ~0%    {5} r2 = JOIN r1 WITH DataFlowImplCommon#4f8df883::Cached::MkTypedContent#fff_20#join_rhs ON FIRST 1 OUTPUT Lhs.1, Lhs.0, Lhs.2, Lhs.3, Rhs.1
         267905  ~0%    {5} r3 = JOIN r2 WITH num#DataFlowImplForHttpClientLibraries#c536b619::TNodeNormal#ff ON FIRST 1 OUTPUT Lhs.2, Lhs.1, Lhs.3, Lhs.4, Rhs.1
         267905  ~0%    {5} r4 = JOIN r3 WITH num#DataFlowImplForHttpClientLibraries#c536b619::TNodeNormal#ff ON FIRST 1 OUTPUT Lhs.3, Lhs.1, Lhs.2, Lhs.4, Rhs.1
        2109240  ~0%    {5} r5 = JOIN r4 WITH DataFlowImplForHttpClientLibraries#c536b619::readProj#2#ff ON FIRST 1 OUTPUT Lhs.3, Lhs.1, Lhs.4, Lhs.2, Rhs.1
                        return r5
```

Author: hvitved

cpp/ql/lib/experimental/semmle/code/cpp/ir/dataflow/internal/DataFlowImpl.qll

@@ -558,13 +558,16 @@ private predicate expectsContentEx(NodeEx n, Content c) {
 pragma[nomagic]
 private predicate notExpectsContent(NodeEx n) { not expectsContentCached(n.asNode(), _) }
 
+pragma[nomagic]
+private predicate readProj(Content c, Configuration config) { read(_, c, _, config) }
+
 pragma[nomagic]
 private predicate store(
   NodeEx node1, TypedContent tc, NodeEx node2, DataFlowType contentType, Configuration config
 ) {
   store(pragma[only_bind_into](node1.asNode()), tc, pragma[only_bind_into](node2.asNode()),
     contentType) and
-  read(_, tc.getContent(), _, config) and
+  readProj(tc.getContent(), config) and
   stepFilter(node1, node2, config)
 }