compiler: edit error messages to avoid -Wformat-diag warnings

compiler: edit error messages to avoid -Wformat-diag warnings

    GCC recently introduced -Wformat-diag to scrutinize GCC error messages.
    It reports a number of warnings about gofrontend code, such as:

    ../../trunk/gcc/go/gofrontend/import.cc: In member function ‘Type* Import::type_for_index(int, const string&, size_t, bool*)’:
    ../../trunk/gcc/go/gofrontend/import.cc:1129:48: warning: unquoted operator ‘>=’ in format [-Wformat-diag]
     1129 |         "error in %s at %lu: bad type index %d >= %d",
          |                                                ^~

    ../../trunk/gcc/go/gofrontend/ast-dump.cc: In member function ‘void Ast_dump_context::dump(Gogo*, const char*)’:
    ../../trunk/gcc/go/gofrontend/ast-dump.cc:203:25: warning: unquoted option name ‘-fgo-dump-ast’ in format [-Wformat-diag]
      203 |     "cannot open %s:%m, -fgo-dump-ast ignored", dumpname.c_str());
          |                         ^~~~~~~~~~~~~

    ../../trunk/gcc/go/gofrontend/expressions.cc: In static member function ‘static Bexpression* Func_expression::get_code_pointer(Gogo*, Named_object*, Location)’:
    ../../trunk/gcc/go/gofrontend/expressions.cc:1350:29: warning: misspelled term ‘builtin function’ in format; use ‘built-in function’ instead [-Wformat-diag]
     1350 |     "invalid use of special builtin function %qs; must be called",
          |                             ^~~~~~~~~~~~~~~~

    ../../trunk/gcc/go/gofrontend/gogo.cc: In member function ‘void Gogo::add_linkname(const string&, bool, const string&, Location)’:
    ../../trunk/gcc/go/gofrontend/gogo.cc:2527:4: warning: unquoted sequence of 2 consecutive punctuation characters ‘//’ in format [-Wformat-diag]
     2527 |   ("%s is not a function; "
          |   ~^~~~~~~~~~~~~~~~~~~~~~~~
     2528 |    "//go:linkname is only supported for functions"),
          |    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    This CL edits error messages to avoid these warnings.

    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/183497

	* go.test/test/blank1.go: Update for diagnostic message changes.

From-SVN: r272608