YAML: Extract the code that skips a comment into a separate method, NFC.

This commit extracts the code that skips over a YAML comment from 
the 'scanToNextToken' method into a separate 'skipComment' method.

This refactoring is motivated by a patch that implements parsing
of YAML block scalars (http://reviews.llvm.org/D9503), as the
method that parses a block scalar reuses the 'skipComment' method.

llvm-svn: 236663