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/cmWriteFileCommand.cxx

@@ -6,6 +6,7 @@
 
 #include "cmExecutionStatus.h"
 #include "cmMakefile.h"
+#include "cmStringAlgorithms.h"
 #include "cmSystemTools.h"
 #include "cm_sys_stat.h"
 
@@ -64,9 +65,9 @@ bool cmWriteFileCommand(std::vector<std::string> const& args,
   cmsys::ofstream file(fileName.c_str(),
                        overwrite ? std::ios::out : std::ios::app);
   if (!file) {
-    std::string error = "Internal CMake error when trying to open file: ";
-    error += fileName;
-    error += " for writing.";
+    std::string error =
+      cmStrCat("Internal CMake error when trying to open file: ", fileName,
+               " for writing.");
     status.SetError(error);
     return false;
   }