Don't insert semicolons in conditions unless indented

In a statement like this
```scala
if foo
    (bar)
then
```
we would never expect a semicolon to be inserted between `foo` and `bar`. The `foo`
would need to be indented itself for this to happen.

So, the following is OK
```scala
if
  val x = ...
  x > 0
then
```
But the following is not:
```scala
if val x = ...
  x > 0
```

Fixes #12757

Author: odersky

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

@@ -72,8 +72,9 @@ object Parsers {
     if source.isSelfContained then new ScriptParser(source)
     else new Parser(source)
 
-  private val InCase: Region => Region = Scanners.InCase.apply
-  private val InCond: Region => Region = Scanners.InBraces.apply
+  private val InCase: Region => Region = Scanners.InCase(_)
+  private val InCond: Region => Region = Scanners.InParens(LPAREN, _)
+  private val InFor : Region => Region = Scanners.InBraces(_)
 
   abstract class ParserCommon(val source: SourceFile)(using Context) {
 
@@ -167,7 +168,7 @@ object Parsers {
   class Parser(source: SourceFile)(using Context) extends ParserCommon(source) {
 
     val in: Scanner = new Scanner(source)
-    // in.debugTokenStream = true    // uncomment to see the token stream of the standard scanner, but not syntax highlighting
+    //in.debugTokenStream = true    // uncomment to see the token stream of the standard scanner, but not syntax highlighting
 
     /** This is the general parse entry point.
      *  Overridden by ScriptParser
@@ -2553,7 +2554,7 @@ object Parsers {
             if (in.token == INDENT)
               inBracesOrIndented(enumerators())
             else {
-              val ts = inSepRegion(InCond)(enumerators())
+              val ts = inSepRegion(InFor)(enumerators())
               if (rewriteToOldSyntax(Span(start)) && ts.nonEmpty)
                 if (ts.head.sourcePos.startLine != ts.last.sourcePos.startLine) {
                   patch(source, Span(forEnd), " {")

tests/pos/i12757.scala

@@ -0,0 +1,8 @@
+val x = Some(10)
+
+def f =
+   if x.exists
+         (x => x == 10) then
+      println("Yes")
+   else
+      println("No")