Fix gcc5 build of ASTMatchersTest.h

After a180d540 the build was failing with:

In file included from /work/llvm.monorepo/clang/unittests/ASTMatchers/ASTMatchersNodeTest.cpp:9:0:
/work/llvm.monorepo/clang/unittests/ASTMatchers/ASTMatchersTest.h:
In function ‘llvm::ArrayRef<clang::TestLanguage> clang::ast_matchers::langCxx11OrLater()’:
/work/llvm.monorepo/clang/unittests/ASTMatchers/ASTMatchersTest.h:64:10:
error: could not convert ‘(const clang::TestLanguage*)(& Result)’ from
‘const clang::TestLanguage*’ to ‘llvm::ArrayRef<clang::TestLanguage>’
   return Result;
          ^