Symbolize with an offset PC on MSVC

For whatever reason this looks to produce more consistent backtraces,
but it's unclear why... For now let's document that we don't know why it
works but otherwise leave it in due to the more accurate
debuginfo-related backtraces it produces.

Author: alexcrichton

src/symbolize/dbghelp.rs

@@ -113,7 +113,11 @@ unsafe fn resolve_with_inline(
         |info| {
             dbghelp.SymFromInlineContextW()(
                 GetCurrentProcess(),
-                frame.AddrPC.Offset,
+                // FIXME: why is `-1` used here and below? It seems to produce
+                // more accurate backtraces on Windows (aka passes tests in
+                // rust-lang/rust), but it's unclear why it's required in the
+                // first place.
+                frame.AddrPC.Offset - 1,
                 frame.InlineFrameContext,
                 &mut 0,
                 info,
@@ -122,7 +126,7 @@ unsafe fn resolve_with_inline(
         |line| {
             dbghelp.SymGetLineFromInlineContextW()(
                 GetCurrentProcess(),
-                frame.AddrPC.Offset,
+                frame.AddrPC.Offset - 1,
                 frame.InlineFrameContext,
                 0,
                 &mut 0,