ASTMatchers: added CXXMethodDecl matcher isPure()

The isPure() CXXMethodDecl matcher matches pure method declaration like "A::x"
in this example:

class A {
  virtual void x() = 0;
}

Patch by Konrad Kleine.

llvm-svn: 202012
diff --git a/clang/unittests/ASTMatchers/ASTMatchersTest.cpp b/clang/unittests/ASTMatchers/ASTMatchersTest.cpp
index cf77f2f..aa3c795 100644
--- a/clang/unittests/ASTMatchers/ASTMatchersTest.cpp
+++ b/clang/unittests/ASTMatchers/ASTMatchersTest.cpp
@@ -1588,6 +1588,13 @@
       methodDecl(isVirtual())));
 }
 
+TEST(Matcher, MatchesPureMethod) {
+  EXPECT_TRUE(matches("class X { virtual int f() = 0; };",
+      methodDecl(isPure(), hasName("::X::f"))));
+  EXPECT_TRUE(notMatches("class X { int f(); };",
+      methodDecl(isPure())));
+}
+
 TEST(Matcher, MatchesConstMethod) {
   EXPECT_TRUE(matches("struct A { void foo() const; };",
                       methodDecl(isConst())));