Ninja: Fix OBJECT_DIR placeholder path conversion

Transform the path using ConvertToNinjaPath just as we do for all other
paths.  This fixes the OutOfSource test for objects in the ../OutOfBinary
directory by computing the proper full path for the /Fd option.
Merge topic 'vs-no-compiler-pdb-setting' into next

87c0d16 Ninja: Fix OBJECT_DIR placeholder path conversion