Revert D60974 "[clang-ifs] Clang Interface Stubs, first version."

Revert D60974 "[clang-ifs] Clang Interface Stubs, first version."

This reverts commit rC363626.

clangIndex depends on clangFrontend. r363626 adds a dependency from
clangFrontend to clangIndex, which creates a circular dependency.

This is disallowed by -DBUILD_SHARED_LIBS=on builds:

    CMake Error: The inter-target dependency graph contains the following strongly connected component (cycle):
      "clangFrontend" of type SHARED_LIBRARY
        depends on "clangIndex" (weak)
      "clangIndex" of type SHARED_LIBRARY
        depends on "clangFrontend" (weak)
    At least one of these targets is not a STATIC_LIBRARY.  Cyclic dependencies are allowed only among static libraries.

Note, the dependency on clangIndex cannot be removed because
libclangFrontend.so is linked with -Wl,-z,defs: a shared object must
have its full direct dependencies specified on the linker command line.

In -DBUILD_SHARED_LIBS=off builds, this appears to work when linking
`bin/clang-9`. However, it can cause trouble to downstream clang library
users. The llvm build system links libraries this way:

    clang main_program_object_file ... lib/libclangIndex.a ...  lib/libclangFrontend.a -o exe

libclangIndex.a etc are not wrapped in --start-group.

If the downstream application depends on libclangFrontend.a but not any
other clang libraries that depend on libclangIndex.a, this can cause undefined
reference errors when the linker is ld.bfd or gold.

The proper fix is to not include clangIndex files in clangFrontend.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@363649 91177308-0d34-0410-b5e6-96231b3b80d8