Add Position-independent Code model Module API.

Summary:
This makes PIC levels a Module flag attribute, which can be queried by the
backend.  The flag is named `PIC Level`, and can have a value of:

  0 - Backend-default
  1 - Small-model (-fpic)
  2 - Large-model (-fPIC)

These match the `-pic-level' command line argument for clang, and the value of the
preprocessor macro `__PIC__'.

Test Plan:
New flags tests specific for the 'PIC Level' module flag.
Tests to be added as part of a future commit for PowerPC, which will use this new API.

Reviewers: rafael, echristo

Reviewed By: rafael, echristo

Subscribers: rafael, llvm-commits

Differential Revision: http://reviews.llvm.org/D5882

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221510 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Support/CodeGen.h

@@ -30,6 +30,10 @@ namespace llvm {
     enum Model { Default, JITDefault, Small, Kernel, Medium, Large };
   }
 
+  namespace PICLevel {
+    enum Level { Default=0, Small=1, Large=2 };
+  }
+
   // TLS models.
   namespace TLSModel {
     enum Model {