[WebAssembly] Improve clang diagnostics for wasm attributes
This patch addresses the review comments on r352930:
- Removes redundant diagnostic checking code
- Removes errnoneous use of diag::err_alias_is_definition, which turned out to be ineffective anyway since functions can be defined later in the translation unit and avoid detection.
- Adds a test for various invalid cases for import_name and import_module.
Differential Revision: https://reviews.llvm.org/D59520