Debug support for global alias variable

Debug support for global alias variable

Starting with (future) Clang 15 (since
https://reviews.llvm.org/D120989), Clang emits the DWARF information
of global alias variables as DW_TAG_imported_declaration.  However,
GDB does not handle it.  It incorrectly always reads this tag as
C++/Fortran imported declaration (type alias, namespace alias and
Fortran module).  This commit adds support to handle this tag as an
alias variable.

This change fixes the failures in the gdb.base/symbol-alias.exp
testcase with current git Clang.  This testcase is also updated to
test nested (recursive) aliases.