Clang docs: add some information about the difference between 'clang' and 'clang -cc1'

This is coming up again and again on the mailing list and IRC.

llvm-svn: 171656

clang/docs/FAQ.rst

@@ -0,0 +1,52 @@
+================================
+Frequently Asked Questions (FAQ)
+================================
+
+.. contents::
+   :local:
+
+Driver
+======
+
+I run ``clang -cc1 ...`` and get weird errors about missing headers
+-------------------------------------------------------------------
+
+Given this source file:
+
+.. code-block:: c
+
+  #include <stdio.h>
+
+  int main() {
+    printf("Hello world\n");
+  }
+
+
+If you run:
+
+.. code-block:: console
+
+  $ clang -cc1 hello.c
+  hello.c:1:10: fatal error: 'stdio.h' file not found
+  #include <stdio.h>
+           ^
+  1 error generated.
+
+``clang -cc1`` is the frontend, ``clang`` is the :doc:`driver
+<DriverInternals>`.  The driver invokes the frontend with options appropriate
+for your system.  To see these options, run:
+
+.. code-block:: console
+
+  $ clang -### -c hello.c
+
+Some clang command line options are driver-only options, some are frontend-only
+options.  Frontend-only options are intended to be used only by developers.
+Users should not run ``clang -cc1`` directly.
+
+If you want to use a frontend-only option ("a ``-cc1`` option"), for example
+``-ast-dump``, then you need to take the ``clang -cc1`` line generated by the
+driver and add the option you need.  Alternatively, you can run
+``clang -Xclang <option> ...`` to force the driver pass ``<option>`` to
+``clang -cc1``.
+

clang/docs/index.rst

@@ -21,6 +21,7 @@ Using Clang as a Compiler
    AddressSanitizer
    ThreadSanitizer
    MemorySanitizer
+   FAQ
 
 Using Clang as a Library
 ========================