[clang][lit] Allow test cases to use the compiler that are used to compile Clang

Required by D83660.
Test cases may want to use the host compiler to compile some mocks for the
test case.

This patch adds two substitutions `%host_cc` and `%host_cxx` to use the host
compilers set via variable `CMAKE_C_COMPILER` and `CMAKE_CXX_COMPILER`.

Patch by Ella Ma!

Reviewed By: steakhal

Differential Revision: https://reviews.llvm.org/D98918