Ugly ugly hack for libstdc++-4.6 and libstdc++-4.7 compatibility. These

libraries have an incorrect definition of std::common_type (inherited from a
bug in the standard -- see LWG issue 2141), whereby they produce reference
types when they should not.

If we instantiate a typedef named std::common_type<...>::type, which is defined
in a system header as decltype(... ? ... : ...), and the decltype produces a
reference type, convert it to the non-reference type. (This doesn't affect any
LWG2141-conforming implementation of common_type, such as libc++'s, because the
default implementation of common_type<...>::type isn't supposed to produce a
reference type.)

This is horrible. I'm really sorry. :( Better ideas appreciated!

llvm-svn: 166455