[C++11] Clarify in the Compiler.h support macros and comments (and
assert it with an #error) that we require MSVC 2012; MSVC 2010 will no
longer suffice.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202631 91177308-0d34-0410-b5e6-96231b3b80d8