Improve documentation of the 'a' specifier and the '<abi>:<pref>' align pair.

llvm-svn: 198636
diff --git a/llvm/lib/IR/DataLayout.cpp b/llvm/lib/IR/DataLayout.cpp
index ee2b4bc..c9e95b4 100644
--- a/llvm/lib/IR/DataLayout.cpp
+++ b/llvm/lib/IR/DataLayout.cpp
@@ -287,6 +287,9 @@
       // Bit size.
       unsigned Size = Tok.empty() ? 0 : getInt(Tok);
 
+      assert((AlignType != AGGREGATE_ALIGN || Size == 0) &&
+             "These specifications don't have a size");
+
       // ABI alignment.
       Split = split(Rest, ':');
       unsigned ABIAlign = inBytes(getInt(Tok));