[libunwind] Don't store a predecremented PC when using SEH

This fixes unwinding in boundary cases on ARM with SEH.

In the case of ARM/Thumb, disp->ControlPc points at the following
instruction, with the thumb bit set. Thus by decrementing 1,
it still points at the next instruction. To achieve the desired
effect of pointing at the previous instruction, one first has to strip
out the thumb bit, then do the decrement by 1 to reach the previous
instruction.

When libcxxabi looks for call site ranges, it already does
`_Unwind_GetIP(context) - 1` (in `scan_eh_tab` in
libcxxabi/src/cxa_personality.cpp), so we shouldn't do the
corresponding `- 1` multiple times.

In the case of libcxxabi on Thumb, `funcStart` (still in `scan_eh_tab`)
may have the thumb bit set. If the program counter address is
decremented both in libunwind (first removing the thumb bit, then
decremented), and then libcxxabi decrements it further, and compares
with a `funcStart` with the thumb bit set, it could point to one byte
before the start of the call site.

Thus: This modification makes libunwind with SEH work with libcxxabi
on Thumb, in settings where libunwind and libcxxabi worked fine with
Dwarf before.

For existing cases with libunwind with SEH (on x86_64 and aarch64),
this modification doesn't break any of my testcases.

Differential Revision: https://reviews.llvm.org/D126869

libunwind/src/Unwind-seh.cpp

@@ -104,7 +104,7 @@ _GCC_specific_handler(PEXCEPTION_RECORD ms_exc, PVOID frame, PCONTEXT ms_ctx,
   if (!ctx) {
     __unw_init_seh(&cursor, disp->ContextRecord);
     __unw_seh_set_disp_ctx(&cursor, disp);
-    __unw_set_reg(&cursor, UNW_REG_IP, disp->ControlPc - 1);
+    __unw_set_reg(&cursor, UNW_REG_IP, disp->ControlPc);
     ctx = (struct _Unwind_Context *)&cursor;
 
     if (!IS_UNWINDING(ms_exc->ExceptionFlags)) {

libunwind/src/UnwindCursor.hpp

@@ -518,6 +518,14 @@ private:
   pint_t getLastPC() const { return _dispContext.ControlPc; }
   void setLastPC(pint_t pc) { _dispContext.ControlPc = pc; }
   RUNTIME_FUNCTION *lookUpSEHUnwindInfo(pint_t pc, pint_t *base) {
+#ifdef __arm__
+    // Remove the thumb bit; FunctionEntry ranges don't include the thumb bit.
+    pc &= ~1U;
+#endif
+    // If pc points exactly at the end of the range, we might resolve the
+    // next function instead. Decrement pc by 1 to fit inside the current
+    // function.
+    pc -= 1;
     _dispContext.FunctionEntry = RtlLookupFunctionEntry(pc,
                                                         &_dispContext.ImageBase,
                                                         _dispContext.HistoryTable);