[clang][ASTImporter] Add import support for SourceLocExpr.

It is possible that imported `SourceLocExpr` can cause not expected behavior (if `__builtin_LINE()` is used together with `__LINE__` for example) but still it may be worth to import these because some projects use it.

Reviewed By: teemperor

Differential Revision: https://reviews.llvm.org/D98876