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.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@202012 91177308-0d34-0410-b5e6-96231b3b80d8