Source sweep: Use cmStrCat for string concatenation

This patch is generated by a python script that uses regular expressions to
search for string concatenation patterns of the kind

```
std::string str = <ARG0>;
str += <ARG1>;
str += <ARG2>;
...
```

and replaces them with a single `cmStrCat` call

```
std::string str = cmStrCat(<ARG0>, <ARG1>, <ARG2>, ...);
```

If any `<ARGX>` is itself a concatenated string of the kind

```
a + b + c + ...;
```

then `<ARGX>` is split into multiple arguments for the `cmStrCat` call.

If there's a sequence of literals in the `<ARGX>`, then all literals in the
sequence are concatenated and merged into a single literal argument for
the `cmStrCat` call.

Single character strings are converted to single char arguments for
the `cmStrCat` call.

`std::to_string(...)` wrappings are removed from `cmStrCat` arguments,
because it supports numeric types as well as string types.

`arg.substr(x)` arguments to `cmStrCat` are replaced with
`cm::string_view(arg).substr(x)`

Source/cmTargetSourcesCommand.cxx

@@ -75,9 +75,8 @@ std::vector<std::string> cmTargetSourcesCommand::ConvertToAbsoluteContent(
       absoluteSrc = src;
     } else {
       changedPath = true;
-      absoluteSrc = this->Makefile->GetCurrentSourceDirectory();
-      absoluteSrc += "/";
-      absoluteSrc += src;
+      absoluteSrc =
+        cmStrCat(this->Makefile->GetCurrentSourceDirectory(), '/', src);
     }
     absoluteContent.push_back(absoluteSrc);
   }