[llvm-pdbutil] Fix broken '-modi' option after change D122226.

The change described by:

https://reviews.llvm.org/D122226

Moved some llvm-pdbutil functionality to the debug PDB library.

This patch addresses a broken '-modi' argument handling, which
causes an assertion if its value is other than '0' or '1'.

In addition, it moves the assertion for the number of occurrences
of the '-modi' argument from the PDB library into the llvm-pdbutil
driver.

Reviewed By: zequanwu

Differential Revision: https://reviews.llvm.org/D123483