swift234

[serialization] Fix potential fall-off-the-end issues.

The way llvm::BitstreamCursor works is that reading an end-of-block
code will pop the current block off the stack, along with all its
record abbreviation codes. This means we’d no longer know how to
read any records. Fix this by telling the cursor not to auto-pop blocks
any time we’re doing a tentative read.

Swift SVN r6135