When building a conditional operator where one operand is a throw-expression

When building a conditional operator where one operand is a throw-expression
and the other is a glvalue of class type, don't forget to copy-initialize a
temporary when performing the lvalue-to-rvalue conversion on the glvalue.
Strangely, DefaultLvalueConversions misses this part of the lvalue-to-rvalue
conversions.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@161450 91177308-0d34-0410-b5e6-96231b3b80d8