PR25368: Replace workaround for build failure with modules enabled with a fix

for the root cause. The 'using llvm::isa;' declaration in Basic/LLVM.h only
pulls the declarations of llvm::isa that were declared prior to it into
namespace clang. In a modules build, this is a hermetic set of just the
declarations from LLVM. In a non-modules build, we happened to also pull the
declaration from lib/CodeGen/Address.h into namespace clang, which made the
code in question accidentally compile.

llvm-svn: 252211