BitcodeReader: Require clients to read the block info block at most once.

This change makes it the client's responsibility to call ReadBlockInfoBlock()
at most once. This is in preparation for a future change that will allow
there to be multiple block info blocks.

See also: http://lists.llvm.org/pipermail/llvm-dev/2016-October/106512.html

Differential Revision: https://reviews.llvm.org/D26016

llvm-svn: 285350

llvm/lib/Bitcode/Reader/BitstreamReader.cpp

@@ -319,9 +319,9 @@ void BitstreamCursor::ReadAbbrevRecord() {
 }
 
 bool BitstreamCursor::ReadBlockInfoBlock() {
-  // If this is the second stream to get to the block info block, skip it.
+  // We expect the client to read the block info block at most once.
   if (getBitStreamReader()->hasBlockInfoRecords())
-    return SkipBlock();
+    report_fatal_error("Duplicate read of block info block");
 
   if (EnterSubBlock(bitc::BLOCKINFO_BLOCK_ID)) return true;