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/ffi.rs

@@ -1182,6 +1182,7 @@ extern "C" {
     pub fn LLVMRustAddByValAttr(Fn: &Value, index: c_uint, ty: &Type);
     pub fn LLVMRustAddStructRetAttr(Fn: &Value, index: c_uint, ty: &Type);
     pub fn LLVMRustAddFunctionAttribute(Fn: &Value, index: c_uint, attr: Attribute);
+    pub fn LLVMRustEmitUWTableAttr(Fn: &Value, async_: bool);
     pub fn LLVMRustAddFunctionAttrStringValue(
         Fn: &Value,
         index: c_uint,