Add an explicit move constructor to SrcBuffer

MSVC can't synthesize the explicit one.  Instead it tries to emit a copy
ctor which would call the deleted copy ctor of unique_ptr.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216244 91177308-0d34-0410-b5e6-96231b3b80d8