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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@198636 91177308-0d34-0410-b5e6-96231b3b80d8

lib/IR/DataLayout.cpp

@@ -287,6 +287,9 @@ void DataLayout::parseSpecifier(StringRef Desc) {
       // 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));