RFR(S) 8214450 Use a higher-level function to store Strings into CDS archive

[Ioi Lam]

http://cr.openjdk.java.net/~iklam/jdk12/8214450-cleanup-string-archive.v01/
https://bugs.openjdk.java.net/browse/JDK-8214450

This is a clean up that simplifies the archiving of shared strings. Also:
    - removed java_lang_String::set_value_raw which has become dead code.
    - fixed a bug in 
HeapShared::check_closed_archive_heap_region_object_class,
      and renamed/commented this function for clarity.

All CDS tests passed locally. Running hs-tiers{1-4} just for sanity.