Add Clang dependency to the check for Clang C++ headers.

The problem is that CMake is mostly imperative and the result of
processing "if (TARGET blah)" checks depends on the order of import of
CMake files.
In this case, "projects" folder is registered before "tools",
and calling "CheckClangHeaders" [renamed to have a better name]
errors out without even giving Clang a chance to be built.

This, in turn, leads to libFuzzer bot failures in some circumstances on
some machines (depends on whether LIT or UNIT tests are scheduled
first).

Differential Revision: https://reviews.llvm.org/D37126

llvm-svn: 311733

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