Minor fix for signedness of asuint bitcast

[petermcneeleychromium]

The Result of a bitcast via 'asuint' should be unsigned.

Code that will (incorrectly) generate an assert.

RWByteAddressBuffer v : register(u0);
void foo() {
  int v_1 = int(0);
  v.InterlockedAdd(int(16u), asint((asuint(int(0)) - asuint(int(123)))), v_1);
  int x = v_1;
}

Failing code (6199) '/dxc/tools/clang/lib/AST/ExprConstant.cpp'

  assert(SI.isSigned() == E->getType()->isSignedIntegerOrEnumerationType() &&
           "Invalid evaluation result.");

[s-perron]

You might want to add a test to https://github.com/microsoft/DirectXShaderCompiler/tree/4fcf67f78f7d6ffd286316112694a3ae000860e2/tools/clang/test/SemaHLSL. That checks the AST has the correct type.

[s-perron]

/AzurePipelines run

[azure-pipelines]

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