Add negative test for SR-12548.

Test SR-12548: `SILCombiner::visitPartialApplyInst` rewrites `partial_apply`
with `@convention(method)` callee to `thin_to_thick_function`.

This produces a SIL verification error: `thin_to_thick_function` only supports
`@convention(thin)` operands.

Author: dan-zheng

test/AutoDiff/compiler_crashers/sr12548-siloptimizer-rewrite-partial-apply-convention-method.swift

@@ -0,0 +1,27 @@
+// RUN: not %target-build-swift -O %s
+
+// SR-12548: SIL verification error regarding
+// `CapturePropagation::rewritePartialApply` for `partial_apply` with
+// `@convention(method)` callee.
+
+import _Differentiation
+
+protocol Protocol: Differentiable {
+  @differentiable
+  func method() -> Self
+}
+
+extension Protocol {
+  @differentiable
+  func method() -> Self { self }
+}
+
+struct Struct: Protocol {}
+
+let _: @differentiable (Struct) -> Struct = { $0.method() }
+
+// SIL verification failed: operand of thin_to_thick_function must be thin: opFTy->getRepresentation() == SILFunctionType::Representation::Thin
+// Verifying instruction:
+//      // function_ref specialized Protocol.method()
+//   %5 = function_ref @$s7crasher8ProtocolPAAE6methodxyFAA6StructV_TG5 : $@convention(method) (@in_guaranteed Struct) -> @out Struct // user: %6
+// ->   %6 = thin_to_thick_function %5 : $@convention(method) (@in_guaranteed Struct) -> @out Struct to $@callee_guaranteed (@in_guaranteed Struct) -> @out Struct // user: %11