Preserve argument indexes when inlining MIR

We store argument indexes on VarDebugInfo. Unlike the previous method of
relying on the variable index to know whether a variable is an argument,
this survives MIR inlining.

We also no longer check if var.source_info.scope is the outermost scope.
When a function gets inlined, the arguments to the inner function will
no longer be in the outermost scope. What we care about though is
whether they were in the outermost scope prior to inlining, which we
know by whether we assigned an argument index.

tests/codegen/inline-function-args-debug-info.rs

@@ -0,0 +1,20 @@
+// This test checks that debug information includes function argument indexes even if the function
+// gets inlined by MIR inlining. Without function argument indexes, `info args` in gdb won't show
+// arguments and their values for the current function.
+
+// compile-flags: -Zinline-mir=yes -Cdebuginfo=2 --edition=2021
+
+#![crate_type = "lib"]
+
+pub fn outer_function(x: usize, y: usize) -> usize {
+    inner_function(x, y) + 1
+}
+
+#[inline]
+fn inner_function(aaaa: usize, bbbb: usize) -> usize {
+    // CHECK: !DILocalVariable(name: "aaaa", arg: 1
+    // CHECK-SAME: line: 14
+    // CHECK: !DILocalVariable(name: "bbbb", arg: 2
+    // CHECK-SAME: line: 14
+    aaaa + bbbb
+}