Enable -mergefunc-use-aliases

If the Rust LLVM fork is used, enable the -mergefunc-use-aliases
flag, which will create aliases for merged functions, rather than
inserting a call from one to the other.

A number of codegen tests needed to be adjusted, because functions
that previously fell below the thunk limit are now being merged.
Merging is prevented either using -C no-prepopulate-passes, or by
making the functions non-identical.

I expect that this is going to break something, somewhere, because
it isn't able to deal with aliases properly, but we won't find out
until we try :)

This fixes #52651.

src/rustllvm/RustWrapper.cpp

@@ -533,6 +533,14 @@ extern "C" uint32_t LLVMRustVersionMinor() { return LLVM_VERSION_MINOR; }
 
 extern "C" uint32_t LLVMRustVersionMajor() { return LLVM_VERSION_MAJOR; }
 
+extern "C" bool LLVMRustIsRustLLVM() {
+#ifdef LLVM_RUSTLLVM
+  return 1;
+#else
+  return 0;
+#endif
+}
+
 extern "C" void LLVMRustAddModuleFlag(LLVMModuleRef M, const char *Name,
                                       uint32_t Value) {
   unwrap(M)->addModuleFlag(Module::Warning, Name, Value);