add a new pop_back_val method which returns the value popped.  This is heretical from a STL standpoint, but is oh-so-useful for things that  can't throw exceptions when copied, like, well, everything in LLVM.
add a new pop_back_val method which returns the value popped.  This is
heretical from a STL standpoint, but is oh-so-useful for things that 
can't throw exceptions when copied, like, well, everything in LLVM.


git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@60587 91177308-0d34-0410-b5e6-96231b3b80d8

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