[Reg 4.14->4.15] CMake build configurations are locked to specific build type

There are several ways how to run into this, for example:

1. Do a build of your application on the command line, with some CMAKE_BUILD_TYPE
2. Open the project in Qt Creator and configure some build configuration(s)
3. Change the build directory of some build configuration to the existing build

Expected: The build directory is taken as "the whole truth", Qt Creator successfully parses the project

Actual: If the existing build was done with a different CMAKE_BUILD_TYPE than the implicit build type that was set for the build configuration, for example if you did a RelWithDebInfo build and point the Debug build configuration to that build directory, parsing the project fails with the non-descriptive warning "project was not successfully parsed".

Qt Creator should accept the new build directory as the new truth about the build configuration. If the new build directory has some build type, the build configuration should just have that build type. I realize that for multi-config generators that is not unique - so for multi-config generators the build configuration should have a visible, user-choosable build type property instead of the current hidden one.