Fixed a serious bug in DeportType where the types could retain DeclContexts that

pointed into the artificial function constructed for the expression.  I now make
anything that pointed to the function as its DeclContext be global while the
copy occurs; afterward I restored the old DeclContext.

Added a testcase that make sure that this works properly and doesn't crash
anything.

<rdar://problem/21049838>

llvm-svn: 241695