C++: Workaround for problem with memcpy flow

The type of the source argument to `memcpy` is `void *`, and somehow
that meant that the copied object itself got type `void`. Since that has
size 0, the SSA construction did not model it as reading from the last
write.

This is probably not the right fix, but maybe it's good enough for now.
The right fix would ensure that the type reported by
`hasOperandMemoryAccess` is `UnknownType`.

When `DefaultTaintTracking.qll` is enabled, this commit has the effect
of restoring a lost results:

    --- a/cpp/ql/test/query-tests/Security/CWE/CWE-119/semmle/tests/OverflowDestination.expected
    +++ b/cpp/ql/test/query-tests/Security/CWE/CWE-119/semmle/tests/OverflowDestination.expected
    @@ -1 +1,2 @@
     | overflowdestination.cpp:30:2:30:8 | call to strncpy | To avoid overflow, this operation should be bounded by destination-buffer size, not source-buffer size. |
    +| overflowdestination.cpp:46:2:46:7 | call to memcpy | To avoid overflow, this operation should be bounded by destination-buffer size, not source-buffer size. |

Author: jbj

cpp/ql/src/semmle/code/cpp/ir/implementation/aliased_ssa/internal/AliasedSSA.qll

@@ -26,7 +26,7 @@ private predicate hasResultMemoryAccess(
     type = languageType.getIRType() and
     isIndirectOrBufferMemoryAccess(instr.getResultMemoryAccess()) and
     (if instr.hasResultMayMemoryAccess() then isMayAccess = true else isMayAccess = false) and
-    if exists(type.getByteSize())
+    if type.getByteSize() > 0
     then endBitOffset = Ints::add(startBitOffset, Ints::mul(type.getByteSize(), 8))
     else endBitOffset = Ints::unknown()
   )
@@ -43,7 +43,7 @@ private predicate hasOperandMemoryAccess(
     type = languageType.getIRType() and
     isIndirectOrBufferMemoryAccess(operand.getMemoryAccess()) and
     (if operand.hasMayReadMemoryAccess() then isMayAccess = true else isMayAccess = false) and
-    if exists(type.getByteSize())
+    if type.getByteSize() > 0
     then endBitOffset = Ints::add(startBitOffset, Ints::mul(type.getByteSize(), 8))
     else endBitOffset = Ints::unknown()
   )