Modules: Rename ModuleBuildFailed => DisableGeneratingGlobalModuleIndex, NFC

Rename CompilerInstance's ModuleBuildFailed field to
DisableGeneratingGlobalModuleIndex, which more precisely describes its
role. Otherwise, it's hard to suss out how it's different from
ModuleLoader::HadFatalFailure, and what sort of code simplifications are
safe.

Differential Revision: https://reviews.llvm.org/D101670