PR18962: Fix "Unable to find record layout information for type"

Clang is using llvm::StructType::isOpaque() as a way of signaling if
we've finished record type conversion in
CodeGenTypes::isRecordLayoutComplete().  However, Clang was setting the
body of the type before it finished laying out the type as a base type.
Laying out the %class.C.base LLVM type attempts to convert more types,
eventually recursively attempting to layout 'C' again, at which point we
would say that layout was complete, even though we were still in the
middle of it.

By not setting the body, we correctly signal that layout is not
complete, and things work as expected.

At some point, it might be worth refactoring this to avoid looking at
the LLVM IR types under construction.