Move 'convertUTF8Sequence' helper into the C++ section of the header file.

It's annotated as "inline", but ConvertUTF.c should still be able to compile
as C89.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173376 91177308-0d34-0410-b5e6-96231b3b80d8