Non-unique logical locations for duplicate function signatures

[sungshik]

Describe the bug

If two function declarations at different physical locations have the same signature (but possibly different bodies), then they have the same logical location. This can lead to internal bookkeeping issues in Typepal (e.g., #2879). See this comment for details.

To Reproduce

data D = foo();
int f(foo()) = 5;
int f(foo()) = 6;

[sungshik]

Possibly related: #2883