[clang][CGExprConstant] handle unary negation on integrals

Consider the statement:
    int x = -1;
And the following AST:
    `-VarDecl 0x55c4823a7670 <x.c:2:1, col:10> col:5 x 'int' cinit
      `-UnaryOperator 0x55c4823a7740 <col:9, col:10> 'int' prefix '-'
        `-IntegerLiteral 0x55c4823a7720 <col:10> 'int' 1

Return the evaluation of the subexpression negated.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D156378

Author: nickdesaulniers

clang/lib/CodeGen/CGExprConstant.cpp

@@ -1363,6 +1363,13 @@ class ConstExprEmitter :
     return Visit(E->getSubExpr(), T);
   }
 
+  llvm::Constant *VisitUnaryMinus(UnaryOperator *U, QualType T) {
+    if (llvm::Constant *C = Visit(U->getSubExpr(), T))
+      if (auto *CI = dyn_cast<llvm::ConstantInt>(C))
+        return llvm::ConstantInt::get(CGM.getLLVMContext(), -CI->getValue());
+    return nullptr;
+  }
+
   // Utility methods
   llvm::Type *ConvertType(QualType T) {
     return CGM.getTypes().ConvertType(T);