Disable Clang's type checking for MPI_2INT

Type checking for MPI_2INT is commented out because Clang 3.3 flags
struct {int i1; int i2;} as different from int[2]. But actually these
two types are of the same layout. Clang gives a type mismatch warning
for a definitely correct code like the following:
 int in[2], out[2];
 MPI_Reduce(in, out, 1, MPI_2INT, MPI_MAXLOC, 0, MPI_COMM_WORLD);

So, we disable type checking for MPI_2INT until Clang fixes this bug.

Fixes #1993

Signed off by Rajeev Thakur <thakur@mcs.anl.gov>

src/include/mpi.h.in

@@ -168,7 +168,21 @@ static const MPI_Datatype mpich_mpi_float_int       MPICH_ATTR_TYPE_TAG_LAYOUT_C
 static const MPI_Datatype mpich_mpi_double_int      MPICH_ATTR_TYPE_TAG_LAYOUT_COMPATIBLE(struct mpich_struct_mpi_double_int)      = MPI_DOUBLE_INT;
 static const MPI_Datatype mpich_mpi_long_int        MPICH_ATTR_TYPE_TAG_LAYOUT_COMPATIBLE(struct mpich_struct_mpi_long_int)        = MPI_LONG_INT;
 static const MPI_Datatype mpich_mpi_short_int       MPICH_ATTR_TYPE_TAG_LAYOUT_COMPATIBLE(struct mpich_struct_mpi_short_int)       = MPI_SHORT_INT;
-static const MPI_Datatype mpich_mpi_2int            MPICH_ATTR_TYPE_TAG_LAYOUT_COMPATIBLE(struct mpich_struct_mpi_2int)            = MPI_2INT;
+
+/*
+ * The MPI_2INT line is commented out because currently Clang 3.3 flags
+ * struct {int i1; int i2;} as different from int[2]. But actually these
+ * two types are of the same layout. Clang gives a type mismatch warning
+ * for a definitely correct code like the following:
+ *  int in[2], out[2];
+ *  MPI_Reduce(in, out, 1, MPI_2INT, MPI_MAXLOC, 0, MPI_COMM_WORLD);
+ *
+ * So, we disable type checking for MPI_2INT until Clang fixes this bug.
+ */
+
+/* static const MPI_Datatype mpich_mpi_2int            MPICH_ATTR_TYPE_TAG_LAYOUT_COMPATIBLE(struct mpich_struct_mpi_2int)            = MPI_2INT
+ */
+
 #if @MPI_LONG_DOUBLE_INT@ != 0x0c000000
 static const MPI_Datatype mpich_mpi_long_double_int MPICH_ATTR_TYPE_TAG_LAYOUT_COMPATIBLE(struct mpich_struct_mpi_long_double_int) = MPI_LONG_DOUBLE_INT;
 #endif