Implement the initial part of C++0x [expr.const]p2, which specifies

that the unevaluated subexpressions of &&, ||, and ? : are not
considered when determining whether the expression is a constant
expression. Also, turn the "used in its own initializer" warning into
a runtime-behavior warning, so that it doesn't fire when a variable is
used as part of an unevaluated subexpression of its own initializer.

Fixes PR9999.

llvm-svn: 131968