Use std::string_view for macro bodies (#1326)

This removes the last use of `strdup`

This required making a lot of related pointers be `const`.
That in turn conflicted with the need to `munmap()` a pointer
eventually, which was similar to the need to eventually `free()`
an `Expansion`'s contents, so I used the same solution of a
`union`. That lets us normally use the `const` pointer for
`const` correctness, and the non-`const` one for the not-really-
mutating destruction cases.

include/platform.hpp

@@ -14,11 +14,6 @@
 # include <strings.h>
 #endif
 
-// MSVC has deprecated strdup in favor of _strdup
-#ifdef _MSC_VER
-# define strdup _strdup
-#endif
-
 // MSVC prefixes the names of S_* macros with underscores,
 // and doesn't define any S_IS* macros; define them ourselves
 #ifdef _MSC_VER