Reverting size_type for the containers from size_type to unsigned.

Various places in LLVM assume that container size and count are unsigned
and do not use the container size_type. Therefore they break compilation
(or possibly executation) for LP64 systems where size_t is 64 bit while
unsigned is still 32 bit.

If we'll ever that many items in the container size_type could be made
size_t for a specific containers after reviweing its other uses.

llvm-svn: 211353