Make sure TypeCheck has a non-deleted default ctor

...as needed by

> core/compilerplugins/clang/check.cxx:19:19: error: call to implicitly-deleted default constructor of 'loplugin::TypeCheck'
>         ? *this : TypeCheck();
>                   ^
> core/compilerplugins/clang/check.hxx:76:5: note: explicitly defaulted function was implicitly deleted here
>     TypeCheck() = default;
>     ^
> core/compilerplugins/clang/check.hxx:78:27: note: default constructor of 'TypeCheck' is implicitly deleted because field 'type_' of const-qualified type 'const clang::QualType' would not be initialized
>     clang::QualType const type_;
>                           ^

after https://llvm.org/svn/llvm-project/cfe/trunk@315194 "Make SourceLocation,
QualType and friends have constexpr constructors" defaulted the clang::QualType
ctor.

Change-Id: Idaeaa4499cea877fd5d66c18b8fce24b7808ba04

compilerplugins/clang/check.hxx

@@ -75,7 +75,7 @@ public:
 private:
     TypeCheck() = default;
 
-    clang::QualType const type_;
+    clang::QualType const type_{};
 };
 
 class DeclCheck {