Add a test for option from config file.

Author: abidh

flang/test/Driver/Inputs/config-7.cfg

@@ -0,0 +1 @@
+-Werror

flang/test/Driver/fcc_override.f90

@@ -1,5 +1,6 @@
 ! RUN: env FCC_OVERRIDE_OPTIONS="#+-Os +-Oz +-O +-O3 +-Oignore +a +b +c xb Xa Omagic ^-###  " %flang -target x86_64-unknown-linux-gnu %s -O2 b -O3 2>&1 | FileCheck %s
 ! RUN: env FCC_OVERRIDE_OPTIONS="x-Werror +-g" %flang -target x86_64-unknown-linux-gnu -Werror %s -c -### 2>&1 | FileCheck %s -check-prefix=RM-WERROR
+! RUN: env FCC_OVERRIDE_OPTIONS="x-Werror" %flang --config=%S/Inputs/config-7.cfg -### %s -c  2>&1 | FileCheck %s -check-prefix=CONF
 
 ! CHECK: "-fc1"
 ! CHECK-NOT: "-Oignore"
@@ -10,3 +11,7 @@
 ! RM-WERROR-NEXT: ### Deleting argument -Werror
 ! RM-WERROR-NEXT: ### Adding argument -g at end
 ! RM-WERROR-NOT: "-Werror"
+
+! Test that FCC_OVERRIDE_OPTIONS does not affect the options from config files.
+! CONF: ### FCC_OVERRIDE_OPTIONS: x-Werror
+! CONF: "-Werror"