[AVR] Add support for the 'interrupt' and 'naked' attributes

Summary:
This teaches clang how to parse and lower the 'interrupt' and 'naked'
attributes.

This allows interrupt signal handlers to be written.

Reviewers: aaron.ballman

Subscribers: malcolm.parsons, cfe-commits

Differential Revision: https://reviews.llvm.org/D28451

llvm-svn: 294402
diff --git a/clang/test/Sema/avr-interrupt-attr.c b/clang/test/Sema/avr-interrupt-attr.c
new file mode 100644
index 0000000..2dfc72a
--- /dev/null
+++ b/clang/test/Sema/avr-interrupt-attr.c
@@ -0,0 +1,8 @@
+// RUN: %clang_cc1 %s -triple avr-unknown-unknown -verify -fsyntax-only
+struct a { int b; };
+
+struct a test __attribute__((interrupt)); // expected-warning {{'interrupt' attribute only applies to functions}}
+
+__attribute__((interrupt(12))) void foo(void) { } // expected-error {{'interrupt' attribute takes no arguments}}
+
+__attribute__((interrupt)) void food() {}