[Runtime] Fix swift_slowAlloc to respect its alignMask parameter.

Instead of calling malloc, call AlignedAlloc. That calls posix_memalign on platforms where it's available. The man page cautions to use it judiciously, but Apple OSes and Linux implement it to call through to malloc when the alignment is suitable. Presumably/hopefully other OSes do the same.

rdar://problem/22975669

Author: mikeash

stdlib/public/runtime/Heap.cpp

@@ -23,12 +23,11 @@
 using namespace swift;
 
 void *swift::swift_slowAlloc(size_t size, size_t alignMask) {
-  // FIXME: use posix_memalign if alignMask is larger than the system guarantee.
-  void *p = malloc(size);
+  void *p = AlignedAlloc(size, alignMask + 1);
   if (!p) swift::crash("Could not allocate memory.");
   return p;
 }
 
 void swift::swift_slowDealloc(void *ptr, size_t bytes, size_t alignMask) {
-  free(ptr);
+  AlignedFree(ptr);
 }