Moved contents of clang_cflags into cflags

    Currently, clang_cflags are appended to cflags.
    Clang_cflags will be deleted from Soong soon.

Test: Treehugger & m nothing
Bug: 226636335
Ignore-AOSP-First: change is already in aosp

Change-Id: I96492f526873c272ce36ea428b7c0073a9395450