[CMake] Ensure correct extension for llvm-lit is used on Windows when...

[CMake] Ensure correct extension for llvm-lit is used on Windows when LLVM_INSTALL_UTILS is enabled.

D77110 initially added support for setting LLVM_CONFIG_DEFAULT_EXTERNAL_LIT
to llvm-lit in the install directory if LLVM_INSTALL_UTILS is on.

D79144 ensured that, on Windows, llvm-lit.py is correctly set for
LLVM_CONFIG_DEFAULT_EXTERNAL_LIT within the context of the build area,
however, it did not account for the install area which is the latter set
directive for this same variable.

This patch ensures that LLVM_CONFIG_DEFAULT_EXTERNAL_LIT under the install
area uses llvm-lit.py under Windows since llvm-lit without the extension
is not created.

Differential Revision: https://reviews.llvm.org/D124197