Diagnostics for conflicting function definitions within a module

Previously, multiple definitions of identical functions within a module were not
recognized, although they have the same mangling. This was problematic because a
binary cannot contain multiple definitions of a symbol, which caused undefined
behavior depending on the compiler backend.

DMD will now raise an error message if there are conflicting implementations
within a single module:

---
void foo() {}
void foo() {} // error
---

Multiple declarations are still allowed as long as there is at most one definition:

---
void bar(int);
void bar(int) { }
void bar(int);
---

DMD will issue a deprecation for mangling schemes that don't support overloading
(`extern(C|Windows|System)`):

---
extern(C):
void foo(int) { }
void foo(double) { } // deprecation
---

This deprecation will become an error in 2.105.