Fix regressions in asSeenFrom introduced in 3.7

The body of this method used to read:

    pre.isStable || !ctx.phase.isTyper

but was changed to drop the second condition in #21954 (originally included in
3.6.4-RC1, reverted from 3.6.4 final but back in 3.7). This has led to a
number of regressions, the last ones discussed in #23423.

To make the testcases added in #21594 pass, this PR proposes a less drastic
change: relax isLegalPrefix as before, unless we're doing an implicit search.
This should dramatically reduce the possibility for regressions.

Author: smarter

compiler/src/dotty/tools/dotc/core/TypeOps.scala

@@ -125,7 +125,9 @@ object TypeOps:
   }
 
   def isLegalPrefix(pre: Type)(using Context): Boolean =
-    pre.isStable
+    // isLegalPrefix is relaxed after typer unless we're doing an implicit
+    // search (this matters when doing summonInline in an inline def like in tests/pos/i17222.8.scala).
+    pre.isStable || !ctx.phase.isTyper && ctx.mode.is(Mode.ImplicitsEnabled)
 
   /** Implementation of Types#simplified */
   def simplify(tp: Type, theMap: SimplifyMap | Null)(using Context): Type = {

tests/pos-macros/i23423/A_1.scala

@@ -0,0 +1,16 @@
+package pkg
+
+import scala.quoted.*
+
+trait HasElem {
+  type Elem
+  type Alias = Elem
+}
+
+object Macro:
+  inline def foo: Unit = ${fooImpl}
+  def fooImpl(using Quotes): Expr[Unit] =
+    '{
+      val lll: (he: HasElem) => he.Alias =
+        (hx: HasElem) => ???
+    }

tests/pos-macros/i23423/B_2.scala

@@ -0,0 +1,6 @@
+object Test:
+  def test: Unit = pkg.Macro.foo
+  // used to be error:
+  // Found:    (hx: pkg.HasElem) => hx.Elem
+  // Required: (he: pkg.HasElem) => he.Elem
+