[clang][ASTImporter] fix variable inline of CXX17 (#87314)

Fix crash in the testcase from
https://github.com/llvm/llvm-project/issues/75114#issuecomment-1872595956


Forget to set inline of variable declaration would make
`isThisDeclarationADefinition` get incorrect result and didn't get
imported variable. This will lead to a new `VarTemplateDecl` being
created and call `setDescribedVarTemplate` again which produces the
crash.

Co-authored-by: huqizhi <836744285@qq.com>