[clang][AST] Set correct DeclContext in ASTImporter lookup table for template params.

Template parameters are created in ASTImporter with the translation unit as DeclContext.
The DeclContext is later updated (by the create function of template classes).
ASTImporterLookupTable was not updated after these changes of the DC. The patch
adds update of the DeclContext in ASTImporterLookupTable.

Reviewed By: martong

Differential Revision: https://reviews.llvm.org/D103792