Compute msymbol hash codes in parallel
    
    This is for the msymbol_hash and msymbol_demangled_hash hashtables
    in objfile_per_bfd_storage. This basically computes those hash
    codes together with the demangled symbol name in the background,
    before it inserts the symbols in the hash table.
    
    gdb/ChangeLog:
    
    2019-09-30  Christian Biesinger  <cbiesinger@google.com>
    
    	* minsyms.c (add_minsym_to_hash_table): Use a previously computed
    	hash code if possible.
    	(add_minsym_to_demangled_hash_table): Likewise.
    	(minimal_symbol_reader::install): Compute the hash codes for msymbol
    	on the background thread.
    	* symtab.h (struct minimal_symbol) <hash_value, demangled_hash_value>:
    	Add these fields.