Fix crash on end-of-file after \ in a char literal, fixes PR14369.

This makes LexCharConstant() look more like LexStringLiteral(), which doesn't
have this bug. Add tests for eof after \ for several other cases.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@168269 91177308-0d34-0410-b5e6-96231b3b80d8