[PATCH] Add PIC-level support to Clang.

[Rafael Espíndola]

> rafael wrote:
>> Don't you want to produce an error if -pic-level is not 1 or 2?
> It's too easy for the user to try to do something like `clang -cc1 -pic-level 3 t.c` and crash the compiler with assert or llvm_unreachable. We should either clamp the level to [0-2] or issue a diagnostic in lib/Frontend/CompilerInvocation.cpp.

I don't think -cc1 options are considered user visible. Having another
value in here means a bug somewhere else in clang.

Cheers,
Rafael