[ASan] Initial support for Kernel AddressSanitizer

This patch adds initial support for the -fsanitize=kernel-address flag to Clang.
Right now it's quite restricted: only out-of-line instrumentation is supported, globals are not instrumented, some GCC kasan flags are not supported.
Using this patch I am able to build and boot the KASan tree with LLVMLinux patches from github.com/ramosian-glider/kasan/tree/kasan_llvmlinux.
To disable KASan instrumentation for a certain function attribute((no_sanitize("kernel-address"))) can be used.

llvm-svn: 240131

clang/lib/CodeGen/CodeGenFunction.cpp

@@ -615,7 +615,7 @@ void CodeGenFunction::StartFunction(GlobalDecl GD,
   }
 
   // Apply sanitizer attributes to the function.
-  if (SanOpts.has(SanitizerKind::Address))
+  if (SanOpts.hasOneOf(SanitizerKind::Address | SanitizerKind::KernelAddress))
     Fn->addFnAttr(llvm::Attribute::SanitizeAddress);
   if (SanOpts.has(SanitizerKind::Thread))
     Fn->addFnAttr(llvm::Attribute::SanitizeThread);