[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