Improve debug symbol names to avoid ambiguity and work better with MSVC's debugger

[dpaoliello]

There are several cases where names of types and functions in the debug info are either ambiguous, or not helpful, such as including ambiguous placeholders (e.g., {{impl}}, {{closure}} or dyn _') or dropping qualifications (e.g., for dynamic types).

Instead, each debug symbol name should be unique and useful:

• Include disambiguators for anonymous DefPathDataName (closures and generators), and unify their formatting when used as a path-qualifier vs item being qualified.
• Qualify the principal trait for dynamic types.
• If there is no principal trait for a dynamic type, emit all other traits instead.
• Respect the qualified argument when emitting ref and pointer types.
• For implementations, emit the disambiguator.
• Print const generics when emitting generic parameters or arguments.

Additionally, when targeting MSVC, its debugger treats many command arguments as C++ expressions, even when the argument is defined to be a symbol name. As such names in the debug info need to be more C++-like to be parsed correctly:

• Avoid characters with special meaning (#, [, ", +).
• Never start a name with < or { as this is treated as an operator.
• >> is always treated as a right-shift, even when parsing generic arguments (so add a space to avoid this).
• Emit function declarations using C/C++ style syntax (e.g., leading return type).
• Emit arrays as a synthetic array$<type, size> type.
• Include a $ in all synthetic types as this is a legal character for C++, but not Rust (thus we avoid collisions with user types).