[clang][deps] Disable global module index

While scanning dependencies of a TU that depends on a PCH, the scanner basically performs mixed implicit/explicit modular compilation. (Explicit modules come from the PCH.) This seems to trip up the global module index.

This patch disables global module index in the dependency scanner.

Reviewed By: Bigcheese

Differential Revision: https://reviews.llvm.org/D118890