pull .ll and .bc writing out of the ASTConsumer destructors into some top

pull .ll and .bc writing out of the ASTConsumer destructors into some top
level code in clang.  This is a cleanup, but does implement "-o" for
-emit-llvm.  One effect of this is that "clang foo.c -emit-llvm" will now
emit into foo.ll instead of stdout.  Use "clang foo.c -emit-llvm -o -" or
"clang < foo.c -emit-llvm" to get the old behavior.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@46791 91177308-0d34-0410-b5e6-96231b3b80d8