backends by removing IMPLICIT_NOTNULL casts from IrStringConcatenation
arguments.
Also fixed an issue where IrStringConcatenation can be lowered into
a null String instead of a literal "null" String if the lone argument
was a platform type String or String with enhanced nullability and the
value was null (e.g., "${FromJava.nullPlatformString()}").
Had to edit some bytecodeText tests to account for the fact that JVM_IR
no longer generates explicit initializations for ConstantValue fields,
but NoConstantValueAttributeForNonConstVals is not the default yet.
Consolidating these into IrStringConcatenations allows the backend to produce efficient code for string concatenations (e.g., using StringBuilder for JVM).
Reuse StringBuilder instances for nested subexpressions.
(NB StringBuilder instance for string template with a string
concatenation inside an expression entry, such as `"${"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
