[ORC] Re-apply r322913 with a fix for a read-after-free error.

ExternalSymbolMap now stores the string key (rather than using a StringRef),
as the object file backing the key may be removed at any time.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@323001 91177308-0d34-0410-b5e6-96231b3b80d8
17 files changed