Fix assertion when try is used inside catch(...) block

Fix assertion when try is used inside catch(...) block

Current assert wiht /EHa:
A single unwind edge may only enter one EH pad
  invoke void @llvm.seh.try.begin()
          to label %invoke.cont1 unwind label %catch.dispatch2

Current IR:
%1 = catchpad within %0 [ptr null, i32 0, ptr null]
   invoke void @llvm.seh.try.begin()
           to label %invoke.cont5 unwind label %catch.dispatch2

The problem is the invoke to llvm.seh.try.begin() missing "funclet"

Accodring: https://llvm.org/docs/LangRef.html#ob-funclet
If any "funclet" EH pads have been entered but not exited (per the
description in the EH doc), it is undefined behavior to execute a
call or invoke.

To fix the problem, when emit seh_try_begin,  call EmitSehTryScopeBegin,
instead of calling EmitRuntimeCallOrInvoke for proper "funclet"
gerenration.

Differential Revision: https://reviews.llvm.org/D150340