Reuse StringBuilder instances for nested subexpressions.
(NB StringBuilder instance for string template with a string
concatenation inside an expression entry, such as `"${"a" + "b"}"`,
will not be reused, although that doesn't seem to be a real-life issue).
#KT-18558 Fixed Target versions 1.1.4
#KT-13682 Fixed Target versions 1.1.4
Join adjacent strings literals, escaped strings, and constant values
(in a language version that supports const val inlining).
Use StringBuilder#append(char) for single-character constants
(e.g., " " in "$a $b").
#KT-17280 Fixed Target versions 1.1.4
#KT-15235 Fixed Target versions 1.1.4
Dmitry Petrov