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.
Ideally we would not expose the keyword for non-CHERI but that is a
rather more invasive change.

clang/lib/Parse/ParseDecl.cpp

@@ -888,8 +888,14 @@ void Parser::ParseNullabilityTypeSpecifiers(ParsedAttributes &attrs) {
 void Parser::ParseCapabilityQualifier(ParsedAttributes &Attrs) {
   IdentifierInfo *AttrName = Tok.getIdentifierInfo();
   SourceLocation AttrNameLoc = Tok.getLocation();
-  Attrs.addNew(AttrName, AttrNameLoc, nullptr, AttrNameLoc, nullptr, 0,
-               ParsedAttr::AS_Keyword);
+  // Report an error if the target does not support CHERI.
+  // TODO: we should not treat __capability as a keyword for non-CHERI.
+  // See https://github.com/CTSRD-CHERI/llvm-project/issues/706.
+  if (!getTargetInfo().SupportsCapabilities())
+    Diag(AttrNameLoc, diag::err_attribute_unsupported) << AttrName << "CHERI";
+  else
+    Attrs.addNew(AttrName, AttrNameLoc, nullptr, AttrNameLoc, nullptr, 0,
+                 ParsedAttr::AS_Keyword);
 }
 
 static bool VersionNumberSeparator(const char Separator) {

clang/lib/Sema/SemaType.cpp

@@ -8333,6 +8333,7 @@ static void HandleCHERICapabilityAttr(QualType &CurType, TypeProcessingState &st
     llvm_unreachable("Unknown type attribute location");
   }
 
+  assert(S.Context.getTargetInfo().SupportsCapabilities());
   CurType = S.BuildPointerInterpretationAttr(CurType, PIK_Capability,
                                              attr.getLoc());
 }

clang/test/Parser/capability-attribute-non-cheri.c

@@ -0,0 +1,8 @@
+// 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 earlier.
+// RUN: %clang_cc1 -triple riscv64 -fsyntax-only %s -verify=no-cheri
+// RUN: %clang_cc1 -triple riscv64 -target-feature +xcheri -fsyntax-only %s -verify=cheri
+// cheri-no-diagnostics
+
+void *__capability foo; // no-cheri-error{{'__capability' attribute is not supported on targets missing CHERI; specify an appropriate -march= or -mcpu=}}