[lldb][DWARFASTParserClang] GetClangDeclForDIE: don't create VarDecl for...

[lldb][DWARFASTParserClang] GetClangDeclForDIE: don't create VarDecl for static data members (#77155)

With DWARFv5, C++ static data members are represented as
`DW_TAG_variable`s (see `faa3a5ea`).

In GetClangDeclForDIE, when trying to parse the `DW_AT_specification`
that a static data member's CU-level `DW_TAG_variable` points to, we
would try to `CreateVariableDeclaration`. Whereas previously it was a
no-op (for `DW_TAG_member`s). However, adding `VarDecls` to RecordDecls
for static data members should always be done in
`CreateStaticMemberVariable`. The test-case is an exapmle where we would
crash if we tried to create a `VarDecl` from within `GetClangDeclForDIE`
for a static data member.

This patch simply checks whether the `DW_TAG_variable` being parsed is a
static data member, and if so, trivially returns from
`GetClangDeclForDIE` (as we previously did for `DW_TAG_member`s).