Fix lifetime issue for backing APValue of OpaqueValueExpr in recursive
constexpr function evaluation, and corresponding ASan / valgrind issue in
tests, by storing the corresponding value with the relevant stack frame. This
also prevents re-evaluation of the source of the underlying OpaqueValueExpr,
which makes a major performance difference for certain contrived code (see
testcase update).

git-svn-id: 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed