fixed valgrind issues of prior commit, this change applies r169456 changes...

fixed valgrind issues of prior commit, this change applies r169456 changes back to the tree with fixes. on darwin no valgrind issues exist in the tests that used to fail.

original change description:

change MCContext to work on the doInitialization/doFinalization model

reviewed by Evan Cheng <evan.cheng@apple.com>

llvm-svn: 169553