librustc_lexer: Simplify "single_quoted_string" method
librustc_lexer: Simplify "single_quoted_string" method

graydon/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust