Auto merge of #38482 - est31:i128, r=eddyb

i128 and u128 support

Brings i128 and u128 support to nightly rust, behind a feature flag. The goal of this PR is to do the bulk of the work for 128 bit integer support. Smaller but just as tricky features needed for stabilisation like 128 bit enum discriminants are left for future PRs.

Rebased version of  #37900, which in turn was a rebase + improvement of #35954 . Sadly I couldn't reopen #37900 due to github. There goes my premium position in the homu queue...

[plugin-breaking-change]

cc #35118 (tracking issue)

src/rustllvm/RustWrapper.cpp

@@ -1246,6 +1246,23 @@ extern "C" void LLVMRustSetLinkage(LLVMValueRef V,
   LLVMSetLinkage(V, from_rust(RustLinkage));
 }
 
+// Returns true if both high and low were successfully set. Fails in case constant wasn’t any of
+// the common sizes (1, 8, 16, 32, 64, 128 bits)
+extern "C" bool LLVMRustConstInt128Get(LLVMValueRef CV, bool sext, uint64_t *high, uint64_t *low)
+{
+    auto C = unwrap<llvm::ConstantInt>(CV);
+    if (C->getBitWidth() > 128) { return false; }
+    APInt AP;
+    if (sext) {
+        AP = C->getValue().sextOrSelf(128);
+    } else {
+        AP = C->getValue().zextOrSelf(128);
+    }
+    *low = AP.getLoBits(64).getZExtValue();
+    *high = AP.getHiBits(64).getZExtValue();
+    return true;
+}
+
 extern "C" LLVMContextRef LLVMRustGetValueContext(LLVMValueRef V) {
   return wrap(&unwrap(V)->getContext());
 }