Unterminated quote error when '<<' used in string literals broken into multiple lines

This is a recent regression caused by a0cb1001.

Reproduction:

# Ok, prints 'a>>bc'
echo 'a>>b\
      c'

# Fails with 'unterminated quote in script', should print 'a<<bc'
echo 'a<<b\
      c'

Build information:

rustc -V: rustc 1.34.0-nightly (02c4c2892 2019-02-26)

git rev-parse HEAD: 3faaeb1f