[lldb][NFC] Remove ClangASTContext::Clear

We now only use this function directly after initialization. As Clear()
resets the ASTContext back to its initial state, this is just a no-op.
There are no other users for this and we no longer can set the ASTContext
after construction, so Clear has no useful purpose anymore. It's also
mostly copy-pasted from Finalize().

llvm-svn: 373460