Fix a bug in the emission of __real/__imag l-values on scalar operands.

Fix a bug in the emission of __real/__imag l-values on scalar operands.
Fix a bug in the emission of complex compound assignment l-values.
Introduce a method to emit an expression whose value isn't relevant.
Make that method evaluate its operand as an l-value if it is one.
Fixes our volatile compliance in C++.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@120931 91177308-0d34-0410-b5e6-96231b3b80d8