Tweak java getlitch not to skip zero (scala#18491)

The "skip zero" behavior was to accommodate escaped EOL in a text block,
but also skips zero-valued escaped char `\0`.

This fix is less precious and uses a flag called `skip`.

Scala 2 previously received the complementary fix, because it wasn't
preserving the char before the escaped EOL (after backporting the dotty
code):
scala/scala#10024

compiler/src/dotty/tools/dotc/parsing/JavaScanners.scala

@@ -439,6 +439,7 @@ object JavaScanners {
         }
         oct.asInstanceOf[Char]
       end octal
+      var skip = false
       def greatEscape: Char =
         nextChar()
         if '0' <= ch && ch <= '7' then octal
@@ -455,11 +456,12 @@ object JavaScanners {
             case '\\' => '\\'
             case CR | LF if inTextBlock =>
               if !scanOnly then nextChar()
+              skip = true
               0
             case _    =>
               if !scanOnly then error("invalid escape character", charOffset - 1)
               ch
-          if x != 0 then nextChar()
+          if !skip then nextChar()
           x
       end greatEscape
 
@@ -470,7 +472,7 @@ object JavaScanners {
           val res = ch
           nextChar()
           res
-      if c != 0 && !scanOnly then putChar(c)
+      if !skip && !scanOnly then putChar(c)
     end getlitch
 
     /** Read a triple-quote delimited text block, starting after the first three double quotes.

tests/run/t12290.check

@@ -64,3 +64,11 @@ XY
 ====
 X Y
 ====
+582059
+====
+00
+====
+2a
+====
+c3bf
+====

tests/run/t12290/Test.scala

@@ -30,4 +30,16 @@ object Test extends App {
   println("====")
   println(valueOf[TextBlocks.Octal.type])
   println("====")
+  println(hexdump(valueOf[TextBlocks.Octal.type]))
+  println("====")
+  println(hexdump(valueOf[TextBlocks.Zero.type].toString))
+  println("====")
+  println(hexdump(valueOf[TextBlocks.Magic.type].toString))
+  println("====")
+  println(hexdump(valueOf[TextBlocks.Maxie.type].toString))
+  println("====")
 }
+
+def hexdump(s: String) = s.getBytes(io.Codec.UTF8.charSet) // java.nio.charset.StandardCharsets.UTF_8
+                          .map(b => f"${b & 0xff}%02x")
+                          .mkString

tests/run/t12290/TextBlocks.java

@@ -81,4 +81,7 @@ class TextBlocks {
 """;
 
     final static String Octal = "X\040Y";
+    final static char Zero = '\0';
+    final static char Magic = '\52';
+    final static char Maxie = '\377';
 }