Put all code in import suggestions under a try-catch

There was a val that escaped before, which caused cyclic references raised
during importSuggestions to be reported instead of being ignored.

Author: odersky

compiler/src/dotty/tools/dotc/typer/ImportSuggestions.scala

@@ -158,7 +158,7 @@ trait ImportSuggestions:
     // Candidates that are already available without explicit import because they
     // are already provided by the context (imported or inherited) or because they
     // are in the implicit scope of `pt`.
-    val alreadyAvailableCandidates: Set[Symbol] = {
+    lazy val alreadyAvailableCandidates: Set[Symbol] = {
       val wildProto = wildApprox(pt)
       val contextualCandidates = ctx.implicits.eligible(wildProto)
       val implicitScopeCandidates = ctx.run.nn.implicitScope(wildProto).eligible

tests/neg/i11994.check

@@ -1,12 +1,8 @@
--- [E046] Cyclic Error: tests/neg/i11994.scala:3:18 --------------------------------------------------------------------
+-- [E008] Not Found Error: tests/neg/i11994.scala:2:28 -----------------------------------------------------------------
+2 |implicit def foo[T <: Tuple.meow]: Unit = ??? // error
+  |                      ^^^^^^^^^^
+  |                      type meow is not a member of object Tuple
+-- [E008] Not Found Error: tests/neg/i11994.scala:3:18 -----------------------------------------------------------------
 3 |given [T <: Tuple.meow]: Unit = ??? // error
-  |                  ^
-  |                  Cyclic reference involving method given_Unit
-  |
-  |                  The error occurred while trying to compute the signature of given instance given_Unit
-  |                    which required to compute the signature of type T
-  |                    which required to compute the signature of given instance given_Unit
-  |
-  |                   Run with both -explain-cyclic and -Ydebug-cyclic to see full stack trace.
-  |
-  | longer explanation available when compiling with `-explain`
+  |            ^^^^^^^^^^
+  |            type meow is not a member of object Tuple