Bitcode: Prepare to move bitcode readers to free functions.
Bitcode: Prepare to move bitcode readers to free functions.

Make initStream() a free function, and change BitcodeReaderBase ctor to take
a BitstreamCursor.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@286622 91177308-0d34-0410-b5e6-96231b3b80d8

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