Add --unwindlib=[libgcc|compiler-rt] to parallel --rtlib=
Summary:
"clang++ hello.cc --rtlib=compiler-rt"
now works without specifying additional unwind or exception
handling libraries.
Reviewers: rsmith
Subscribers: srhines, dberris, aheejin, llvm-commits
Differential Revision: https://reviews.llvm.org/D57128
llvm-svn: 352404