Emit an error instead of crashing on invalid use of __capability

Previously, attempting to generate code that contained __capability while
using a target without support for CHERI would assert with
`Assertion `llvm::isPowerOf2_32(Align) && "Alignment must be power of 2"' failed.`
Fix this by emitting a more user-friendly error message instead of crashing
when we encounter __capability but CHERI is not supported.

clang/lib/Sema/SemaType.cpp

@@ -8332,7 +8332,9 @@ static void HandleCHERICapabilityAttr(QualType &CurType, TypeProcessingState &st
   default:
     llvm_unreachable("Unknown type attribute location");
   }
-
+  // Report an error if the target does not support CHERI.
+  if (!S.Context.getTargetInfo().SupportsCapabilities())
+    S.Diag(attr.getLoc(), diag::err_attribute_unsupported) << Name << "CHERI";
   CurType = S.BuildPointerInterpretationAttr(CurType, PIK_Capability,
                                              attr.getLoc());
 }

clang/test/CodeGen/cheri/riscv/non-cheri-march.c

@@ -0,0 +1,11 @@
+// NOTE: Assertions have been autogenerated by utils/update_cc_test_checks.py
+/// Using __capability without the appropriate -march string previously
+/// crashed clang during codegen instead of emitting an error
+// RUN: %clang_cc1 -triple riscv64 -o - -emit-llvm %s -verify=no-cheri
+// RUN: %clang_cc1 -triple riscv64 -target-feature +xcheri -o - -emit-llvm %s | FileCheck %s
+
+// CHECK-LABEL: @test(
+// CHECK-NEXT:  entry:
+// CHECK-NEXT:    ret i8 addrspace(200)* null
+//
+void *__capability test(void) { return 0; } // no-cheri-error{{__capability attribute is not supported on targets missing CHERI; specify an appropriate -march= or -mcpu=}}