Add a JITEventListener interface that gets called back when a new function is

Add a JITEventListener interface that gets called back when a new function is
emitted or the machine code for a function is freed.  Chris mentioned that we
may also want a notification when a stub is emitted, but that'll be a future
change.  I intend to use this to tell oprofile where functions are emitted and
what lines correspond to what addresses.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@74157 91177308-0d34-0410-b5e6-96231b3b80d8