Fix a clash between CONTIGUOUS and SAVE attribute in flang (issue #673)

This simple patch adds CONTIGUOUS attribute to the list of attributes
allowed to use along with the SAVE attribute.

Signed-off-by: Paul Osmialowski <pawel.osmialowski@arm.com>

test/llvm_ir_correct/contig.f90

@@ -0,0 +1,5 @@
+! RUN: %flang -c %s -fsyntax-only
+module contig
+  implicit none
+  integer, dimension(:), pointer, contiguous, save :: ptr
+end module

tools/flang1/flang1exe/semant.c

@@ -292,7 +292,7 @@ static struct {
        ET_B(ET_VALUE) | ET_B(ET_VOLATILE) | ET_B(ET_SHARED) |
        ET_B(ET_ASYNCHRONOUS) | ET_B(ET_PROTECTED) | ET_B(ET_PINNED) |
        ET_B(ET_TEXTURE) | ET_B(ET_DEVICE) | ET_B(ET_MANAGED) |
-       ET_B(ET_IMPL_MANAGED))},
+       ET_B(ET_CONTIGUOUS) | ET_B(ET_IMPL_MANAGED))},
     {"target",
      ~(ET_B(ET_ACCESS) | ET_B(ET_ALLOCATABLE) | ET_B(ET_DIMENSION) |
        ET_B(ET_INTENT) | ET_B(ET_OPTIONAL) | ET_B(ET_SAVE) | ET_B(ET_VALUE) |