forward type alignment information to seqs (#12430)

tests/collections/thardalignmentconstraint.nim

@@ -0,0 +1,17 @@
+discard """
+disabled: true
+"""
+
+# does not yet work
+
+{.passC: "-march=native".}
+
+type
+  m256d {.importc: "__m256d", header: "immintrin.h".} = object
+
+proc set1(x: float): m256d {.importc: "_mm256_set1_pd", header: "immintrin.h".}
+
+for _ in 1..1000:
+  var x = newSeq[m256d](1)
+  x[0] = set1(1.0) # test if operation causes segfault
+  doAssert (cast[uint](x[0].addr) and 31) == 0