[Index] Added a ShouldSkipFunctionBody callback to libIndex, and refactored...

[Index] Added a ShouldSkipFunctionBody callback to libIndex, and refactored clients to use it instead of inventing their own solution

Subscribers: jkorous, arphaman, kadircet, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D66879

llvm-svn: 370338