[AST] Skip verification of typealiases marked as "debugger aliases".

LLDB creates global typealiases that have archetypes in them, which
violates AST invariants and trips up the AST verifier. Introduce a specific
bit that LLDB can set to indicate that a given typealias is such an alias.
We'll skip AST verification for such typealiases.

As a way to stage in this change without requiring synchronization
across the Swift and LLDB repos, also match typealiases with the names
$.*lldb in the AST verifier and skip verification. Once LLDB is
setting the bit appropriately, we'll remove this hack.

lib/AST/Decl.cpp

@@ -2513,6 +2513,7 @@ TypeAliasDecl::TypeAliasDecl(SourceLoc TypeAliasLoc, SourceLoc EqualLoc,
   : GenericTypeDecl(DeclKind::TypeAlias, DC, Name, NameLoc, {}, GenericParams),
     TypeAliasLoc(TypeAliasLoc), EqualLoc(EqualLoc) {
   Bits.TypeAliasDecl.IsCompatibilityAlias = false;
+  Bits.TypeAliasDecl.IsDebuggerAlias = false;
 }
 
 SourceRange TypeAliasDecl::getSourceRange() const {