Sema: Permit array l-values in asm output operands

GCC permits array l-values in asm output operands even though they
aren't modifiable l-values.  We used to permit it but this behavior
regressed in r224916.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@224918 91177308-0d34-0410-b5e6-96231b3b80d8