Auto merge of rust-lang#3716 - cgettys-microsoft:dev/cgettys/process_id_fixup-03, r=RalfJung

Remove GetCurrentProcessId's frame_in_std check

Most of the support required to close rust-lang#1727 was actually added a while back, in rust-lang#2215.

However, for some reason, even though the Unix/Linux syscall equivalent has no `frame_in_std()` check, the Windows `GetCurrentProcessId` check did. While the vast majority of use cases use `std::process::id`, there's no particular reason to penalize any Windows code that is no_std or for whatever other reason choses to call the function directly (e.g. via the generated [windows-sys](https://docs.rs/windows-sys/latest/windows_sys/Win32/System/Threading/fn.GetCurrentProcessId.html) method). The emulation should still work fine. Given there's no reason not to, we might as well simplify the code a tiny bit and save that branch / frame check during runtime too.

This PR removes the `frame_in_std` restriction for `GetCurrentProcessId`, and also moves it into the environment related shim section per discussion in rust-lang/miri#1727 (comment).

Still passes existing tests/pass/getpid.rs test.

Closes rust-lang#1727 unless we wish to give a dummy value when isolated, which we don't seem to want to do at this time.

Author: bors

src/tools/miri/src/shims/windows/foreign_items.rs

@@ -138,6 +138,11 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
                 let result = this.GetUserProfileDirectoryW(token, buf, size)?;
                 this.write_scalar(result, dest)?;
             }
+            "GetCurrentProcessId" => {
+                let [] = this.check_shim(abi, Abi::System { unwind: false }, link_name, args)?;
+                let result = this.GetCurrentProcessId()?;
+                this.write_int(result, dest)?;
+            }
 
             // File related shims
             "NtWriteFile" => {
@@ -743,11 +748,6 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
                 // Any non zero value works for the stdlib. This is just used for stack overflows anyway.
                 this.write_int(1, dest)?;
             }
-            "GetCurrentProcessId" if this.frame_in_std() => {
-                let [] = this.check_shim(abi, Abi::System { unwind: false }, link_name, args)?;
-                let result = this.GetCurrentProcessId()?;
-                this.write_int(result, dest)?;
-            }
             // this is only callable from std because we know that std ignores the return value
             "SwitchToThread" if this.frame_in_std() => {
                 let [] = this.check_shim(abi, Abi::System { unwind: false }, link_name, args)?;