clang-format: [JS] Support regex literals with trailing escaped slash.

Before:
  var regex = / a\//; int i;

After:
  var regex = /a\//;
  int i;

This required pushing the Lexer into its wrapper class and generating a
new one in this specific case. Otherwise, the sequence get lexed as a
//-comment. This is hacky, but I don't know a better way (short of
supporting regex literals in the Lexer).

Pushing the Lexer down seems to make all the call sites simpler.

llvm-svn: 217444