Clarify struct usage guidelines
Clarify struct usage guidelines

The current coding standards restrict the use of struct to PODs, but no
one has been following them.  This patch updates the standards to
clarify when structs are dangerous and describe common practice in LLVM.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@202728 91177308-0d34-0410-b5e6-96231b3b80d8

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