Add LLVMGetAttrKindIDInContext in the C API in order to facilitate migration away...

Add LLVMGetAttrKindIDInContext 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.

Reviewers: Wallbraker, whitequark, joker.eph, echristo

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D18749

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@266257 91177308-0d34-0410-b5e6-96231b3b80d8