Reserve some constant values for the Swift calling convention.

Swift has a custom calling convention that also requires some new flags
on arguments and one new attribute on alloca instructions. This patch
does not include the implementation of that calling convention - that
will be provided as part of the open-source release of Swift; this only
reserves the bitcode constant values so that they are not used for
other purposes.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@243379 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Bitcode/Writer/BitcodeWriter.cpp

@@ -1902,6 +1902,8 @@ static void WriteInstruction(const Instruction &I, unsigned InstID,
     assert(AlignRecord < 1 << 5 && "alignment greater than 1 << 64");
     AlignRecord |= AI.isUsedWithInAlloca() << 5;
     AlignRecord |= 1 << 6;
+    // Reserve bit 7 for SwiftError flag.
+    // AlignRecord |= AI.isSwiftError() << 7;
     Vals.push_back(AlignRecord);
     break;
   }