Implement -Wshift-op-parentheses for: a << b + c

This appears to be consistent with GCC's implementation of the same warning
under -Wparentheses. Suppressing a << b + c for cases where 'a' is a user
defined type for compatibility with C++ stream IO. Otherwise suggest
parentheses around the addition or subtraction subexpression.

(this came up when MSVC was complaining (incorrectly, so far as I can tell)
about a perceived violation of this within the LLVM codebase, PR14001)

llvm-svn: 165283