Find a better name for origin coming from free() about memory-sanitizer HOT 6 CLOSED

1. Find space in StackDepot for an "origin type" flag: allocation / 
deallocation / a bunch of custom types (like __msan_poison).
2. Change the origin description in the report to "heap deallocation"
3. Maybe change the report header to MemorySanitizer: use-after-free

This is coming up over and over again.

I understand we are stuck at #1, is this a problem? Maybe instead we could 
check the top PC agains the ranges occupied by __interceptor_free etc?

One possible cheap way to solve this: when we record the deallocation stack 
trace on free(), delete, realloc, etc record the top frame somewhere. 
It will always be one of very few, i.e. we can use a fixed size array. 
Then, when reporting a bug, see if the first frame of the origin stack trace is 
one 
of the deallocation frames. 

Sounds hackish.
There is plenty of space in the stackdepot for "stack type". We use an entire 
uptr for the number of frames in the stack trace!

Also note that realloc() should generate 2 distinct origins, as it both 
allocates and deallocated memory.

Fixed in http://llvm.org/viewvc/llvm-project?view=revision&revision=226821