Add support for multi-line string literals when converting string concatenation to string interpolation

[AppAppWorks]

Description

1551 added a code action for converting string concatenation to string interpolation, however multi-line string literals are not yet supported. We should add support for this, e.g.

"""
I
am
a
""" + value + 
"""
multi-line
string
literal
"""

will be converted to

"""
I
am
a\(value)multi-line
string
literal
"""

[ahoppen]

Synced to Apple’s issue tracker as rdar://131940259