Refactoring in commit v3.4.0-rc1~74^2~1 (MSVC: Rewrite manifest file
handling with Makefile and Ninja, 2015-09-15) broke handling of this
option. Fix it and add a test case.
Classify .manifest sources separately, add dependencies on them, and
pass them to the MS manifest tool to merge with linker-generated
manifest files.
Inspired-by: Gilles Khouzam <gillesk@microsoft.com>