[Concurrency] Don't complain about interpolation variables being mutated.

Implicitly-generated interpolation variables are mutated within the
autoclosures created by a `spawn let`. Don't complain about them being
concurrently accessed, because they aren't.

Fixes rdar://76020473.

Author: DougGregor

lib/Sema/TypeCheckConcurrency.cpp

@@ -1879,6 +1879,10 @@ namespace {
       // Check whether this is a local variable, in which case we can
       // determine whether it was safe to access concurrently.
       if (auto var = dyn_cast<VarDecl>(value)) {
+        // Ignore interpolation variables.
+        if (var->getBaseName() == ctx.Id_dollarInterpolation)
+          return false;
+
         auto parent = mutableLocalVarParent[declRefExpr];
 
         // If the variable is immutable, it's fine so long as it involves

test/decl/var/spawn_let.swift

@@ -38,3 +38,8 @@ func cook() async throws {
   _ = try await veggies
   _ = await meat
 }
+
+func testInterpolation() async {
+  spawn let x = "\(12345)"
+  _ = await x
+}