Merging r323360:

------------------------------------------------------------------------
r323360 | kparzysz | 2018-01-24 19:42:19 +0100 (Wed, 24 Jan 2018) | 2 lines

[Hexagon] Accept lowercase b in -hvx-length=64b and -hvx-length=128b

------------------------------------------------------------------------


git-svn-id: https://llvm.org/svn/llvm-project/cfe/branches/release_60@323769 91177308-0d34-0410-b5e6-96231b3b80d8