[clang][TypePrinter][NFC] Make SuppressInlineNamespaceMode an enum class (#170802)

The enum cases for `SuppressInlineNamespaceMode` leaked into the rest of
`PrintingPolicy`, meaning if we wanted to introduce another enum (which
I'm planning on doing in an unrelated PR), then `All`/`None`/`Redundant`
are all already taken, which are quite useful names.

This patch turns `SuppressInlineNamespaceMode` into an `enum class`,
freeing up the names for use by other future enums in `PrintingPolicy`.

Unfortunately that means we have to cast the values when assigning to
`PrintingPolicy::SuppressInlineNamespace`, because its actual type is
`unsigned`. But if we ever make the bitfields have proper enumeration
types (which AFAIU is dependent on an MSVC codegen fix), then we can get
rid of those casts.

While doing this I found three instances of assigning booleans to
`SuppressInlineNamespace`. I added a FIXME in LLDB to check what the
intended value of the enum should be.