[MCJIT] Add cmake variables to customize ittapi git location and revision.

[MCJIT] Add cmake variables to customize ittapi git location and revision.

To support llorg builds this patch provides the following changes:

1)  Added cmake variable ITTAPI_GIT_REPOSITORY to control the location of ITTAPI repository.
     Default value of ITTAPI_GIT_REPOSITORY is github location: https://github.com/intel/ittapi.git
     Also, the separate cmake variable ITTAPI_GIT_TAG was added for repo tag.
2)  Added cmake variable ITTAPI_SOURCE_DIR to control the place where the repo will be cloned.
     Default value of ITTAPI_SOURCE_DIR is build area: PROJECT_BINARY_DIR

Reviewed By: etyurin, bader

Patch by ekovanov.

Differential Revision: https://reviews.llvm.org/D91935