cmMakefile: Let AddDefinition accept a value as cm::string_view

This changes `cmMakefile::AddDefinition` to take a `cm::string_view` as value
argument instead of a `const char *`.

Benefits are:
- `std::string` can be passed to `cmMakefile::AddDefinition` directly without
  the `c_str()` plus string length recomputation fallback.
- Lengths of literals passed to `cmMakefile::AddDefinition` can be computed at
  compile time.

In various sources uses of `cmMakefile::AddDefinition` are adapted to avoid
`std::string::c_str` calls and the `std::string` is passed directly.
Uses of `cmMakefile::AddDefinition`, where a `nullptr` `const char*` might
be passed to `cmMakefile::AddDefinition` are extended with `nullptr` checks.

Source/cmLoadCommandCommand.cxx

@@ -230,7 +230,7 @@ bool cmLoadCommandCommand::InitialPass(std::vector<std::string> const& args,
   }
 
   // Report what file was loaded for this command.
-  this->Makefile->AddDefinition(reportVar, fullPath.c_str());
+  this->Makefile->AddDefinition(reportVar, fullPath);
 
   // find the init function
   std::string initFuncName = args[0] + "Init";