When checking the encoding of an 8-bit string literal, don't just check the

When checking the encoding of an 8-bit string literal, don't just check the
first codepoint! Also, don't reject empty raw string literals for spurious
"encoding" issues. Also, don't rely on undefined behavior in ConvertUTF.c.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@152344 91177308-0d34-0410-b5e6-96231b3b80d8