runtime: make FuncForPC return the innermost inlined frame

Returning the innermost frame instead of the outermost
makes code that walks the results of runtime.Caller{,s}
still work correctly in the presence of mid-stack inlining.

Fixes #29582

Change-Id: I2392e3dd5636eb8c6f58620a61cef2194fe660a7
Reviewed-on: https://go-review.googlesource.com/c/156364
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/runtime2.go

@@ -663,6 +663,17 @@ type _func struct {
 	nfuncdata uint8   // must be last
 }
 
+// Pseudo-Func that is returned for PCs that occur in inlined code.
+// A *Func can be either a *_func or a *funcinl, and they are distinguished
+// by the first uintptr.
+type funcinl struct {
+	zero  uintptr // set to 0 to distinguish from _func
+	entry uintptr // entry of the real (the "outermost") frame.
+	name  string
+	file  string
+	line  int
+}
+
 // layout of Itab known to compilers
 // allocated in non-garbage-collected memory
 // Needs to be in sync with