[llvm-config] Report --bindir based on LLVM_TOOLS_INSTALL_DIR

Summary:
`LLVM_TOOLS_INSTALL_DIR` was introduced in r272200 in order to override the directory
name into which to install LLVM's executable. However, `llvm-config --bindir` still reported
`$PREFIX/bin` independent of what LLVM_TOOLS_INSTALL_DIR was set to.

This fixes the out-of-tree clang standalone build for me.

Reviewers: beanz, tstellar

Reviewed By: tstellar

Subscribers: chapuni, tstellar, llvm-commits

Differential Revision: https://reviews.llvm.org/D22499

llvm-svn: 304458