Fix to PR15826 - clang hits assert in clang::ASTContext::getASTRecordLayout.
Revert (most of) r185393 and r185395.

"Remove floating point computations form SpillPlacement.cpp."

These commits caused test failures in lencod on clang-native-arm-lnt.

I suspect these changes are only exposing an existing issue, but
reverting anyway to keep the bots passing while we investigate.