[autolink-extract] Support (and ignore) LLVM IR files. (#71142)

In Linux, the current implementation of swift-autolink-extract does not
support LLVM IR files resulting from using LTO.

If one tries to build LLVM using LTO and then try to link one of the
targets that use `swiftc` to link, but link against LLVM object files
(like `swift-plugin-server`), `swift-autolink-extract` will fail saying
that some object files are not valid.

To deal with LLVM IR files correctly, create and pass
a `llvm::LLVMContext` around, which allows the APIs in `llvm::object` to
read LLVM IR files. Additionally, handle the case of `IRObjectFile` when
extracting, but perform no action.

test/AutolinkExtract/llvm-ir.cpp

@@ -0,0 +1,7 @@
+// REQUIRES: autolink-extract
+// RUN: %clang -flto=thin -c -o - %s | %target-swift-autolink-extract -o - - 2>&1 | %FileCheck --allow-empty %s
+// CHECK-NOT: The file was not recognized as a valid object file
+
+int test() {
+  return 1;
+}