add some comments to MCSymbol header, make the ctor private so that only MCContext can create these.
add some comments to MCSymbol header, make the ctor private so that only MCContext can create these.


git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@74590 91177308-0d34-0410-b5e6-96231b3b80d8

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