Add matcher for linkage specification

Patch by Jacques Pienaar.

llvm-svn: 217135
diff --git a/clang/unittests/ASTMatchers/ASTMatchersTest.cpp b/clang/unittests/ASTMatchers/ASTMatchersTest.cpp
index 3fbe896..e074c6e 100644
--- a/clang/unittests/ASTMatchers/ASTMatchersTest.cpp
+++ b/clang/unittests/ASTMatchers/ASTMatchersTest.cpp
@@ -377,6 +377,11 @@
                           hasName("M"), hasDeclContext(namespaceDecl()))))));
 }
 
+TEST(DeclarationMatcher, LinkageSpecification) {
+  EXPECT_TRUE(matches("extern \"C\" { void foo() {}; }", linkageSpecDecl()));
+  EXPECT_TRUE(notMatches("void foo() {};", linkageSpecDecl()));
+}
+
 TEST(ClassTemplate, DoesNotMatchClass) {
   DeclarationMatcher ClassX = classTemplateDecl(hasName("X"));
   EXPECT_TRUE(notMatches("class X;", ClassX));