Xcode: Generate per-target file references (#15111)

Xcode requires a separate PBXFileReference for each target source group
that references a source file.  Xcode 6 now diagnoses re-use of the same
PBXFileReference from multiple source groups.  Add the referencing
target name to our internal map key so we use a per-target reference.
Merge topic 'xcode-duplicate-file-refs' into next

cf92fe2d Xcode: Generate per-target file references (#15111)