Add __has_feature(cheri)

This is intended to replace __has_feature(capabilities) which is almost
certainly too generic to upstream.  We'll support them side by side for
the long term in our tree, but projects should generally migrate to
__has_feature(cheri) as their need for older compilers passes.

Author: brooksdavis

clang/include/clang/Basic/Features.def

@@ -319,6 +319,7 @@ FEATURE(experimental_library, LangOpts.ExperimentalLibrary)
 
 // CHERI features:
 FEATURE(capabilities, PP.getTargetInfo().SupportsCapabilities())
+FEATURE(cheri, PP.getTargetInfo().SupportsCapabilities())
 FEATURE(pointer_interpretation, PP.getTargetInfo().SupportsCapabilities())
 FEATURE(cheri_casts, PP.getTargetInfo().SupportsCapabilities())
 // CHERI typed sealed pointers

clang/test/Preprocessor/cheri-features.c

@@ -5,6 +5,15 @@
 // RUN:   2>&1 | FileCheck --check-prefix=CHECK-MIPS %s
 
 
+#if __has_feature(cheri)
+#pragma message("__has_feature(cheri)")
+// CHECK-CHERI: warning: __has_feature(cheri)
+void* __capability w = 0;
+#else
+#pragma message("no cheri")
+// CHECK-MIPS: warning: no cheri
+#endif
+
 #if __has_feature(capabilities)
 #pragma message("__has_feature(capabilities)")
 // CHECK-CHERI: warning: __has_feature(capabilities)