MCOL-2178 Separate ha_mcs_pushdown.cpp compilation.

Set a proper type for string literals on ConstantColumn ctor
to fix the regression produced by MCOL-174.

Removed OPTIMIZER_SWITCH_EXISTS_TO_IN b/c MDB produces
unsupported optimization with it and CS couldn't create
ExistsFilter.

dbcon/mysql/CMakeLists.txt

@@ -6,6 +6,7 @@ include_directories( ${ENGINE_COMMON_INCLUDES}
 SET ( libcalmysql_SRCS
     ha_mcs_sysvars.cpp
     ha_mcs_client_udfs.cpp
+    ha_mcs_pushdown.cpp
     ha_calpont.cpp
     ha_calpont_impl.cpp
     ha_calpont_dml.cpp