Implement __attribute__((internal_linkage)).

The attrubite is applicable to functions and variables and changes
the linkage of the subject to internal.

This is the same functionality as C-style "static", but applicable to
class methods; and the same as anonymouns namespaces, but can apply
to individual methods of a class.

Following the proposal in
http://lists.llvm.org/pipermail/cfe-dev/2015-October/045580.html

llvm-svn: 252648
diff --git a/clang/test/Sema/internal_linkage.c b/clang/test/Sema/internal_linkage.c
new file mode 100644
index 0000000..f4deccc
--- /dev/null
+++ b/clang/test/Sema/internal_linkage.c
@@ -0,0 +1,21 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s
+
+int var __attribute__((internal_linkage));
+int var2 __attribute__((internal_linkage,common)); // expected-error{{'internal_linkage' and 'common' attributes are not compatible}} \
+                                                   // expected-note{{conflicting attribute is here}}
+int var3 __attribute__((common,internal_linkage)); // expected-error{{'common' and 'internal_linkage' attributes are not compatible}} \
+                                                   // expected-note{{conflicting attribute is here}}
+
+int var4 __attribute__((common)); // expected-error{{'common' and 'internal_linkage' attributes are not compatible}} \
+// expected-note{{previous definition is here}}
+int var4 __attribute__((internal_linkage)); // expected-note{{conflicting attribute is here}} \
+// expected-error{{'internal_linkage' attribute does not appear on the first declaration of 'var4'}}
+
+int var5 __attribute__((internal_linkage)); // expected-error{{'internal_linkage' and 'common' attributes are not compatible}}
+int var5 __attribute__((common)); // expected-note{{conflicting attribute is here}}
+
+__attribute__((internal_linkage)) int f() {}
+struct __attribute__((internal_linkage)) S { // expected-warning{{'internal_linkage' attribute only applies to variables and functions}}
+};
+
+__attribute__((internal_linkage("foo"))) int g() {} // expected-error{{'internal_linkage' attribute takes no arguments}}