PR 12586: Fix assert while running libc++ testsuite: deal with exception

PR 12586: Fix assert while running libc++ testsuite: deal with exception
specifications on member function templates of class templates and other such
nested beasties. Store the function template from which we are to instantiate
an exception specification rather than trying to deduce it. Plus some
additional test cases.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@155076 91177308-0d34-0410-b5e6-96231b3b80d8