MFC r236149:

Pull in r157212 from upstream clang trunk:

Revert r115805. An array type is required to have a range type,
however, the range can be unknown for the upper bound.

Testcase to follow.

Part of rdar://11457152

This should fix ctfconvert producing error messages during kernel
builds, similar to:

ERROR: scsi_all.c: die 24561: failed to retrieve array bounds

These were caused by incorrect debug information for flexible array
members of structs.