[FLANG] Fix MSVC + clang-cl build

Flang build on windows with MSVC environment and clang-cl compiler
requires clang_rt.builtin.${target} library. This patch allows us to
locate and include this link library. This is mostly needed for flang
runtime and associated unittests. Clang implicitly uses this library
while calling lld-link via clang driver however a standalone call to
lld-link doesnt link against clang_rt library on its own.

Differential Revision: https://reviews.llvm.org/D137112