Modules: Simplify how DisableGeneratingGlobalModuleIndex is set, likely NFC

DisableGeneratingGlobalModuleIndex was being set by
CompilerInstance::findOrCompileModuleAndReadAST most of (but not all of)
the times it returned `nullptr` as a "normal" failure. Pull that up to
the caller, CompilerInstance::loadModule, to simplify the code. This
resolves a number of FIXMEs added during the refactoring in
5cca6223.

The extra cases where this is set are all some version of a fatal error,
and the only client of the field, shouldBuildGlobalModuleIndex, seems
to be unreachable in that case. Even if there is some corner case where
this has an effect, it seems like the right/consistent behaviour.

Differential Revision: https://reviews.llvm.org/D101672