Add LLVMGetAttrKindID in the C API in order to facilitate migration away from LLVMAttribute

Summary:
LLVMAttribute has outlived its utility and is becoming a problem for C API users that what to use all the LLVM attributes. In order to help moving away from LLVMAttribute in a smooth manner, this diff introduce LLVMGetAttrKindIDInContext, which can be used instead of the enum values.

See D18749 for reference.

Reviewers: Wallbraker, whitequark, joker.eph, echristo, rafael

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D19081

llvm-svn: 266842