[IRMover] Improve diagnostic messages for conflicting metadata

This does the similar for error messages as rL344011 has done for warnings.

With llvm::lto::LTO, the error might appear when LTO::run() is executed.
In that case, the calling code cannot know which module causes the error
and, subsequently, cannot hint the user.

Differential Revision: https://reviews.llvm.org/D61880

llvm-svn: 360857