[clang] Move `ApplyHeaderSearchOptions` from Frontend to Lex

In D116750, the `clangFrontend` library was added as a dependency of `LexTests` in order to make `clang::ApplyHeaderSearchOptions()` available. This increased the number of TUs the test depends on.

This patch moves the function into `clangLex` and removes dependency of `LexTests` on `clangFrontend`.

Reviewed By: thakis

Differential Revision: https://reviews.llvm.org/D117024