Improve error for assignment to incomplete class.

PR7681.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@189510 91177308-0d34-0410-b5e6-96231b3b80d8
Handle -D arguments ending in a backslash.

We translate these into #define directives; to preserve gcc-compatible
semantics (where the expanded macro includes the backslash), we add
an extra "\\\n" to the end of the synthesized "#define".

<rdar://problem/14810220>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@189511 91177308-0d34-0410-b5e6-96231b3b80d8