Rename AST node matchers to match the AST node names directly. Part of this...

Rename AST node matchers to match the AST node names directly. Part of this rename also splits recordDecl() (which used to match CXXRecordDecl) into recordDecl() (that matches RecordDecl) and cxxRecordDecl (that matches CXXRecordDecl). Also adds isStruct(), isUnion(), and isClass() narrowing matchers for RecordDecl objects.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@247885 91177308-0d34-0410-b5e6-96231b3b80d8