Fix issue 102.

Clang now gets attribute merging between redeclarations right, and this caused a
regression. Now we only force-keep a forward declaration with attributes if the
attributes are explicitly written.

Well-needed test refactoring from vsapsai to extract the test for this out of
badinc.cc into a dedicated test file, fwd_decl_with_attribute.cc.

git-svn-id: http://include-what-you-use.googlecode.com/svn/trunk@480 812d1f90-f944-d88a-411a-122bdc93e1ea