Comment parsing: Simplify Lexer::skipLineStartingDecorations (NFC)

Inspection of the first character can just be handled by the loop as
well, it does exactly the same thing. Dereferencing the pointer a second
time shouldn't be an issue: the middle end can eliminate that second
read as it's separated from the first only by a pure function call.

Reviewed By: gribozavr2

Differential Revision: https://reviews.llvm.org/D116186