libexpr: rename confusing makeImmutableString -> gcCopyStringIfNeeded

The purpose of this function has little to do with immutability. Value's
strings are never mutated, and the point of this function is to
singleton empty strings.

Change-Id: Ifd41dd952409d54e4d3de9ab59064e6928b0e480

src/libexpr/gc-alloc.hh

@@ -7,6 +7,7 @@
 #include <list>
 #include <map>
 #include <new>
+#include <string_view>
 #include <vector>
 
 #if HAVE_BOEHMGC
@@ -116,4 +117,8 @@ inline char * gcAllocString(size_t size)
     return cstr;
 }
 
+/// Returns a C-string copied from @ref toCopyFrom, or a single, static empty
+/// string if @ref toCopyFrom is also empty.
+char const * gcCopyStringIfNeeded(std::string_view toCopyFrom);
+
 }