[CMake] Properly handle the LTO cache arguments for MinGW

We want to make sure that LINKER_IS_LLD_LINK is properly set - in
this case it shouldn't be set when building for MinGW.

Then we want to make the test for it correct and finally include
the option to build with thinlto cache since the MinGW driver now
supports that.

Differential Revision: https://reviews.llvm.org/D80493