SourceManager: Use the same fake SLocEntry whenever it fails to load

Instead of putting a fake `SLocEntry` at `LoadedSLocEntryTable[Index]`
when it fails to load in `SourceManager::loadSLocEntry`, allocate a fake
one. Unless someone is sniffing the address of the returned `SLocEntry`
(doubtful), this won't be a functionality change. Note that
`SLocEntryLoaded[Index]` wasn't being set to `true` either before or
after this change so no accessor is every going to look at
`LoadedSLocEntryTable[Index]`.

As a side effect, drop the `mutable` from `LoadedSLocEntryTable`.

Differential Revision: https://reviews.llvm.org/D89748