Implement a hack intended to allow Clang to parse libstdc++ 4.5's

headers, which use C++0x generalized initializer lists. Per PR7069, it
appears that the only use is as the return type of a function, so this
commit enables this extension just in that narrow case. If it's enough
for libstdc++ 4.5, or if it can be trivially extended to work with
libstdc++ 4.5, we'll keep it. Otherwise, or if this breaks anything,
we'll revert and wait for the real feature.

llvm-svn: 127507