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.cc

@@ -0,0 +1,23 @@
+#include "gc-alloc.hh"
+
+#include <cstring>
+#include <string_view>
+
+namespace nix
+{
+
+char const * gcCopyStringIfNeeded(std::string_view toCopyFrom)
+{
+    if (toCopyFrom.empty()) {
+        return "";
+    }
+
+    size_t const size = toCopyFrom.size();
+    char * cstr = gcAllocString(size + 1);
+    memcpy(cstr, toCopyFrom.data(), size);
+    cstr[size] = '\0';
+
+    return cstr;
+}
+
+}