test: add a scale test for the number of IterableDeclContext getting parsed.

Author: nkcsgexi

validation-test/compiler_scale/nominal_bodies.gyb

@@ -0,0 +1,9 @@
+// RUN: %scale-test --sum-multi --parse --begin 5 --end 16 --step 5 --select NumIterableDeclContextParsed %s
+// REQUIRES: OS=macosx
+// REQUIRES: asserts
+
+struct S${N} {}
+class C${N} {}
+enum E${N} {}
+extension C${N} {}
+protocol P${N} {}