Fix mismatched memory allocation and deallocation.

[s-perron]

In NestedNameSpecifier.cpp, the Append function allocates data using new:

DirectXShaderCompiler/tools/clang/lib/AST/NestedNameSpecifier.cpp

Lines 449 to 450 in e1bb926

	char *NewBuffer =
	new char[NewCapacity]; // HLSL Change: Use overridable operator new

However, if it deallocated using free:

DirectXShaderCompiler/tools/clang/lib/AST/NestedNameSpecifier.cpp

Lines 514 to 516 in e1bb926

	if (BufferCapacity) {
	free(Buffer);
	BufferCapacity = 0;

This can cause problems. I'm changing the free to delete.