llvm: migrate to new parameter-bearing uwtable attr

In https://reviews.llvm.org/D114543 the uwtable attribute gained a flag
so that we can ask for sync uwtables instead of async, as the former are
much cheaper. The default is async, so that's what I've done here, but I
left a TODO that we might be able to do better.

While in here I went ahead and dropped support for removing uwtable
attributes in rustc: we never did it, so I didn't write the extra C++
bridge code to make it work. Maybe I should have done the same thing
with the `sync|async` parameter but we'll see.

compiler/rustc_codegen_llvm/src/llvm/mod.rs

@@ -31,6 +31,10 @@ impl LLVMRustResult {
     }
 }
 
+pub fn EmitUWTableAttr(llfn: &Value, async_: bool) {
+    unsafe { LLVMRustEmitUWTableAttr(llfn, async_) }
+}
+
 pub fn AddFunctionAttrStringValue(llfn: &Value, idx: AttributePlace, attr: &CStr, value: &CStr) {
     unsafe {
         LLVMRustAddFunctionAttrStringValue(llfn, idx.as_uint(), attr.as_ptr(), value.as_ptr())