Add @dllname decorator for custom exported symbol names

[ASDAlexander77]

Summary

• New @dllname("custom_name") decorator sets the symbol an exported function, method, accessor or variable gets in a shared library / DLL. For example @dllname("custom_add") export function add(...) is exported as custom_add instead of M.add.
• Functions, methods and get/set accessors: the decorator becomes a dllname attribute that LowerToLLVM forwards as an LLVM function attribute, and ExportFixPass renames the function (all uses follow the rename).
• Variables: LLVM globals carry no string attributes, so TypeScriptToLLVMLoweringPass renames the global symbol and all its uses before lowering. A dynamic-import copy in an importer is not renamed.
• Importers: DeclarationPrinter emits @dllname(...) into __decls for functions, variables, methods and accessors, and every runtime symbol lookup (dynamic import of functions/variables, static/constructor methods, vtable slots, accessors, indexers) resolves the custom name.

Test plan

• New test-compile-shared-export-import-dllname: renamed function, intra-library call, renamed variable changed through a library function, renamed static/instance methods, renamed getter/setter
• All 268 shared-library ctest tests pass (gc/rc/none, compile and JIT)
• DeclarationPrinterTest.* 59/59 pass, including 5 new dllname cases
• llvm-objdump -p on the built DLL shows custom_add, custom_counter, calc_twice, calc_scale, calc_get_double, calc_set_double and no M.add/M.counter/M.Calc.* entries for them

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