use size_t for GeneratorExpressionContent::ContentLength to fix some warnings

CMake/Source/cmGeneratorExpressionParser.cxx: In member function ‘void cmGeneratorExpressionParser::ParseGeneratorExpression(std::vector<cmGeneratorExpressionEvaluator*>&)’:
CMake/Source/cmGeneratorExpressionParser.cxx:116:55: warning: conversion to ‘unsigned int’ from ‘long int’ may alter its value [-Wconversion]
CMake/Source/cmGeneratorExpressionParser.cxx:240:39: warning: conversion to ‘int’ from ‘long int’ may alter its value [-Wconversion]
Merge topic 'genex-conversion-warnings' into next

a80fe4b use size_t for GeneratorExpressionContent::ContentLength to fix some warnings