[Backtracing] Change the return type of one of the paths functions.

Since it's copying, it may as well return `char *` instead of
`const char *`.

rdar://103071801

include/swift/Runtime/Paths.h

@@ -73,7 +73,7 @@ swift_getRootPath();
 /// \return A string containing the full path to the executable.  This string
 ///         should be released with `free()` when no longer required.
 SWIFT_RUNTIME_EXPORT
-const char *
+char *
 swift_copyAuxiliaryExecutablePath(const char *name);
 
 #endif // SWIFT_RUNTIME_PATHS_H

stdlib/public/runtime/Paths.cpp

@@ -356,7 +356,7 @@ _swift_win32NameFromNTName(LPWSTR pszFilename) {
 } // namespace
 
 SWIFT_RUNTIME_EXPORT
-const char *
+char *
 swift_copyAuxiliaryExecutablePath(const char *name)
 {
   const char *rootPath = swift_getRootPath();