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/cmExecProgramCommand.cxx

@@ -8,6 +8,7 @@
 #include "cmExecutionStatus.h"
 #include "cmMakefile.h"
 #include "cmProcessOutput.h"
+#include "cmStringAlgorithms.h"
 #include "cmSystemTools.h"
 
 typedef cmProcessOutput::Encoding Encoding;
@@ -74,9 +75,8 @@ bool cmExecProgramCommand(std::vector<std::string> const& args,
 
   std::string command;
   if (!arguments.empty()) {
-    command = cmSystemTools::ConvertToRunCommandPath(args[0]);
-    command += " ";
-    command += arguments;
+    command = cmStrCat(cmSystemTools::ConvertToRunCommandPath(args[0]), ' ',
+                       arguments);
   } else {
     command = args[0];
   }
@@ -192,10 +192,7 @@ bool RunCommand(std::string command, std::string& output, int& retVal,
 #else
   std::string commandInDir;
   if (dir) {
-    commandInDir = "cd \"";
-    commandInDir += dir;
-    commandInDir += "\" && ";
-    commandInDir += command;
+    commandInDir = cmStrCat("cd \"", dir, "\" && ", command);
   } else {
     commandInDir = command;
   }