[Target] Protect Processes' language runtimes map with a mutex

Summary:
From what I understand, it's possible for multiple threads to request
a specific language runtime (e.g. CPPLanguageRuntime). This leads to a data
race.

Reviewers: jingham, JDevlieghere, compnerd, clayborg

Differential Revision: https://reviews.llvm.org/D62169

llvm-svn: 361442