implement the alias attirbute (in both Sema and Codegen)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@52092 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/test/CodeGen/attributes.c b/test/CodeGen/attributes.c
index a310a3a..95273f8 100644
--- a/test/CodeGen/attributes.c
+++ b/test/CodeGen/attributes.c
@@ -24,3 +24,7 @@
 // RUN: clang -emit-llvm < %s | grep 't7.*nounwind'
 void t7() __attribute__((noreturn, nothrow));
 void t7() {}
+
+// RUN: clang -emit-llvm < %s | grep 't9.*alias.*weak.*t8'
+void __t8() {}
+void t9() __attribute__((weak, alias("__t8")));