Disallow non-lhs all-underscore variable names

[mlechu]

Addresses part of #57547. Currently, we only show the "all-underscore variable name not allowed" error when the offending symbol is in value position according to compile, but an earlier pass would keep them as raw symbols regardless. This led to raw symbols making their way out of lowering in an edge case.

This change: Reject all-underscore variables unless they're being written to. Also, improve an error message slighly.

Before:

julia> function f(); 1; _; end;
ERROR: syntax: all-underscore identifiers are write-only and their values cannot be used in expressions

julia> function f(); _; 1; end;
ERROR: Found raw symbol in code returned from lowering. Expected all symbols to have been resolved to GlobalRef or slots.

After:

julia> function f(); _; 1; end;
ERROR: syntax: all-underscore identifiers are write-only and their values cannot be used in expressions

Suggestions for more tests are welcome.