Use a RAII guard to control access to DisassemblerLLVMC.

Summary:
This patch replaces the manual lock/unlock calls for gaining exclusive access to the disassembler with
a RAII-powered access scope. This should prevent that we somehow skip over these trailing Unlock calls
(e.g. with early returns).

We also have a second `GetDisasmToUse` method now that takes an already constructed access scope to
prevent deadlocks when we call this from other methods.

Reviewers: #lldb, davide, vsk

Reviewed By: #lldb, davide, vsk

Subscribers: davide, vsk, lldb-commits

Differential Revision: https://reviews.llvm.org/D51319

llvm-svn: 340835