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/cmAddCustomCommandCommand.cxx

@@ -14,6 +14,7 @@
 #include "cmMessageType.h"
 #include "cmPolicies.h"
 #include "cmSourceFile.h"
+#include "cmStringAlgorithms.h"
 #include "cmSystemTools.h"
 #include "cmTarget.h"
 
@@ -202,8 +203,7 @@ bool cmAddCustomCommandCommand(std::vector<std::string> const& args,
             // and later references "${CMAKE_CURRENT_SOURCE_DIR}/out.txt".
             // This is fairly obscure so we can wait for someone to
             // complain.
-            filename = mf.GetCurrentBinaryDirectory();
-            filename += "/";
+            filename = cmStrCat(mf.GetCurrentBinaryDirectory(), '/');
           }
           filename += copy;
           cmSystemTools::ConvertToUnixSlashes(filename);