When multiple actions are specified, the last one is used and others are
overridden. This might lead to confusion if the user is used to driver's
`-S -emit-llvm` behavior.
```
%clang_cc1 -S -emit-llvm a.c # -S is overridden
%clang_cc1 -emit-llvm -S a.c # -emit-llvm is overridden
%clang_cc1 -fsyntax-only -S a.c # -fsyntax-only is overridden
```
However, we want to continue supporting overriding the driver action
with -Xclang:
* `clang -c -Xclang -ast-dump a.c` (`%clang -cc1 -emit-obj ...
-main-file-name a.c ... -ast-dump`)
* `clang -c -xc++ -Xclang -emit-module stl.modulemap`
As an exception, we allow -ast-dump* options to be composed together
(e.g. `-ast-dump -ast-dump-lookups` in AST/ast-dump-lookups.cpp).
