BitcodeWriter: Remove redundant (and incorrect) check for whether to emit module summary.

The function name Module::empty() is slightly misleading in that it
only tests for the presence of functions in the module. However we
still want to emit the module summary if the module contains only
global variables or aliases. The presence of such entities can be
determined simply by checking the summary directly, as we are doing
below.

Differential Revision: http://reviews.llvm.org/D21669

llvm-svn: 273638