NumericLiteralParser::ParseNumberStartingWithZero(): Try to appease MSC16's miscompilation.

Investigating yet. It seems msc16 miscompiles s[1] to be folded.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@191485 91177308-0d34-0410-b5e6-96231b3b80d8