When checking whether we can load a bridging precompiled header (PCH), we end
up parsing module maps. If Clang emits a warning or error while parsing the
module maps we encounter, the Swift compiler would crash because we have not
properly established the invariants of Clang's diagnostic engine. Perform a
pairsed set of BeginSourceFile/EndSourceFile calls on the Clang diagnostic
consumer to set up the appropriate state.
Fixes rdar://problem/57626886.
