Remove ASTContexts from SystemInitializerCommon

Summary:
The AST contexts are not needed in the server components, and the clang context in particular
pulls in large parts of clang into the binary. Simply removing these two calls reduces the
lldb-server size by about 50%--80%, depending on the architecture and build type.

This should not impact the client parts as the same calls are already present in
SystemInitializerFull.

Reviewers: tberghammer, clayborg

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D20236

llvm-svn: 269416