Changed Dockerfiles to install LLVM into /usr/local

Summary:
Previously, the installation path was simply '/'.
Using '/usr/local' would ensure that LLVM installation does not
conflict with software installed via package managers.

Reviewers: mehdi_amini, klimek

Reviewed By: klimek

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D37213

llvm-svn: 311890