[ARM][Driver] Change float-abi warning

[michaelplatings]

Summary

Previously the warning stated "flag ignored" which is only partially
true - the invalid flag would prevent -feature +soft-float-abi from
being emitted which resulted in user-visible behaviour like
__ARM_PCS_VFP being defined.

Rather than attempt to coerce invalid flags into valid behaviour, don't
describe the expected behaviour.

Ideally the warning would be an error, as it is in GCC. However there
are tests in llvm-project that trigger the warning. Therefore one has to
assume that making the warning an error would break other code that
already exists in the wild.

Also apply test improvements suggested by @MaskRay on D150902.

[michaelplatings]

Fix duplicate implicit group warning

clang/include/clang/Basic/DiagnosticDriverKinds.td

 def warn_drv_deprecated_arg : Warning<
   "argument '%0' is deprecated, use '%1' instead">, InGroup<Deprecated>;
 def warn_drv_assuming_mfloat_abi_is : Warning<
   "unknown platform, assuming -mfloat-abi=%0">;
 def warn_drv_unsupported_float_abi_by_lib : Warning<
   "float ABI '%0' is not supported by current library">,
   InGroup<DiagGroup<"unsupported-abi">>;
 def warn_drv_no_floating_point_registers: Warning<
-  "'%0': selected processor lacks floating point registers; flag ignored">,
-  InGroup<OptionIgnored>;
+  "'%0': selected processor lacks floating point registers">,
+  InGroup<DiagGroup<"unsupported-abi">>;
 def warn_ignoring_ftabstop_value : Warning<
   "ignoring invalid -ftabstop value '%0', using default value %1">;
 def warn_drv_overriding_flag_option : Warning<
   "overriding '%0' option with '%1'">,
   InGroup<DiagGroup<"overriding-t-option">>;
 def warn_drv_treating_input_as_cxx : Warning<
   "treating '%0' input as '%1' when in C++ mode, this behavior is deprecated">,
   InGroup<Deprecated>;

clang/test/Driver/arm-no-float-regs.c

-// REQUIRES: arm-registered-target
-
 // Check that -mfloat-abi=hard gives a warning if FP registers aren't available.
-// RUN: %clang --target=arm-none-eabi -mcpu=cortex-m0 -mfloat-abi=hard -### -c %s 2>&1
+// RUN: %clang --target=arm-none-eabi -mcpu=cortex-m0 -mfloat-abi=hard -### -c %s 2>&1 \
+// RUN:   | FileCheck %s
 
 // RUN: %clang --target=arm-none-eabi -mcpu=cortex-m0 -mhard-float -### -c %s 2>&1 \
 // RUN:   | FileCheck -check-prefix=HARDFLOAT %s
 
 // -mfloat-abi=hard and -march=...+nofp are incompatible in this instance:
 // RUN: %clang --target=arm-none-eabi -march=armv8.1-m.main+nofp -mfloat-abi=hard -### -c %s 2>&1
 // -mfloat-abi=hard and -march=...+nofp are compatible in this instance:
 // RUN: %clang --target=arm-none-eabi -march=armv8.1-m.main+mve+nofp -mfloat-abi=hard -### -c %s 2>&1 \
 // RUN:   | FileCheck -check-prefix=NOWARN %s
 
 // Here the float ABI is calculated as "hard" and FP registers are
 // calculated to not be available. Since the float ABI wasn't specified
 // explicitly, the warning should not be emitted.
 // RUN: %clang -target thumbv5-windows -mcpu=arm10tdmi -### -c %s -o /dev/null 2>&1 \
 // RUN:   | FileCheck -check-prefix=NOWARN %s
 
 // CHECK: warning: '-mfloat-abi=hard': selected processor lacks floating point registers
 // HARDFLOAT: warning: '-mhard-float': selected processor lacks floating point registers
-// NOWARN-NOT: selected processor lacks floating point registers
+// NOWARN-NOT: warning: