frontend(wasm): handle collisions of export and global names #1390

Per Wasm spec the following is valid and contains counter as name of both a global and an exported function.

(module $counter_module
  (global $counter (mut i32) (i32.const 0))
  (func $read_counter (export "counter") (result i32)
    global.get $counter))

For now such a collision between global and export name is rejected with a diagnostic (#1357), as we don't expect producers to generate it.

Some notes in case we want/need to support it later on:

• The export cannot be renamed. The export name must become the linkage name for external calls to resolve.
• So the (private) global must be renamed and in HIR the name of the global and references to it must be rewritten.
• The diagnostic is emitted in the wasm frontend in fn resolve_func_symbols.