librustc_lexer: Make "eat_float_exponent" return bool instead of result
librustc_lexer: Make "eat_float_exponent" return bool instead of result

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