Don't create a null reference in ASTUnit::LoadFromCommandLine.

This change is the last in the pack of five commits
(also see r216691, r216694, r216695, and r216696) that reduces the number
of test failures in "check-clang" invocation in UBSan bootstrap
from 2443 down to 5.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@216697 91177308-0d34-0410-b5e6-96231b3b80d8
Better codegen support for DLL attributes being dropped after the first declaration (PR20792)

For the following code:

  __declspec(dllimport) int f(int x);
  int user(int x) {
    return f(x);
  }
  int f(int x) { return 1; }

Clang will drop the dllimport attribute in the AST, but CodeGen would have
already put it on the LLVM::Function, and that would never get updated.
(The same thing happens for global variables.)

This makes Clang check dropped DLL attribute case each time the LLVM object
is referenced.

This isn't perfect, because we will still get it wrong if the function is
never referenced by codegen after the attribute is dropped, but this handles
the common cases and makes us not fail in the verifier.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@216699 91177308-0d34-0410-b5e6-96231b3b80d8
Introduce -DLLVM_USE_SANITIZER=Undefined CMake option to build UBSan-ified version of LLVM/Clang.

I've fixed most of the simple bugs and currently "check-llvm" test suite
has 26 failures, and "check-clang" suite has 5 failures.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@216701 91177308-0d34-0410-b5e6-96231b3b80d8