Merge upstream GCC changes for include/ and libiberty/ directories

This commit pulls in the latest changes for the include/ and
libiberty/ directories.  The last sync was in commit
533da483.

This commit also removes the file libiberty/rust-demangle.h, this file
has been removed in upstream GCC, and should have been deleted as part
of the previous sync up, which included this ChangeLog entry:

  2019-11-16  Eduard-Mihai Burtescu  <eddyb@lyken.rs>

        ....
	* rust-demangle.h: Remove.

I've grep'd over the binutils-gdb source and can find no reference to
the rust-demangle.h file, and everything seems to build fine without
it, so I assume its continued existence was a mistake.

include/ChangeLog:

	Import from gcc mainline:
	2020-02-05  Andrew Burgess  <andrew.burgess@embecosm.com>

        * hashtab.h (htab_remove_elt): Make a parameter const.
        (htab_remove_elt_with_hash): Likewise.

libiberty/ChangeLog:

	* rust-demangle.h: Removed.

	Import from gcc mainline:
	2020-02-05  Andrew Burgess  <andrew.burgess@embecosm.com>

        * hashtab.c (htab_remove_elt): Make a parameter const.
        (htab_remove_elt_with_hash): Likewise.

	2020-01-23  Alexandre Oliva <oliva@adacore.com>

        * argv.c (writeargv): Output empty args as "".

	2020-01-18  Iain Sandoe  <iain@sandoe.co.uk>

	* cp-demangle.c (cplus_demangle_operators): Add the co_await
	operator.
	* testsuite/demangle-expected: Test co_await operator mangling.