[Fuchsia] Include libClang and clang-include-fixer in the toolchain

libClang is used by other Clang based tools such as cquery while
clang-include-fixer is generally a useful tool.

Differential Revision: https://reviews.llvm.org/D43404

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@325665 91177308-0d34-0410-b5e6-96231b3b80d8