[5.9] [AST] add a source location for GenericTypeParamDecls

[QuietMisdreavus]

Cherry-pick of #64688

• Explanation: Functions with a mix of generic type parameters and existential types crash SymbolGraphGen and any other users of SourceEntityWalker.
• Scope: The change introduces a source location for GenericTypeParamDecl, which previously had none. This doesn't seem to affect any consumers of it (i checked SourceKit's cursor info, -dump-ast, and SymbolGraphGen), but the change may affect other internals of the compiler.
• Issue: rdar://105982860
• Risk: Low. The addition of the source location for GenericTypeParamDecl seems to be unused, and this change should not affect normal compilation.
• Testing: An automated test has been added to ensure that SymbolGraphGen's use of SourceEntityWalker no longer trips an assertion on a reproduction. Existing automated tests still pass.
• Reviewer: @slavapestov

[QuietMisdreavus]

@swift-ci Please test