[CodeGen] Stabilize C2/D2 to C1/D1 replacement order

The conversion iterates over CodeGenModule::Replacements (a StringMap)
and replaces C2/D2 and moves C1/D1 (
commit 0196a1d9 in 2013, to make the
output look nicer). The iteration order is not guaranteed to be
deterministic, and may cause destructors.cpp to exhibit different
function orders. Use a MapVector instead.

While here, fix an IWYU issue by adding an explicit include, though
MapVector is already used in CodeGenModule.h.