Add Fuzzer to the list of sanitizers which support coverage.

[george.karpenkov]

Summary

Without this change, additional coverage flags specified after
-fsanitize=fuzzer would get discarded.

lib/Driver/SanitizerArgs.cpp

 
 enum : SanitizerMask {
   NeedsUbsanRt = Undefined | Integer | Nullability | CFI,
   NeedsUbsanCxxRt = Vptr | CFI,
   NotAllowedWithTrap = Vptr,
   RequiresPIE = DataFlow,
   NeedsUnwindTables = Address | Thread | Memory | DataFlow,
   SupportsCoverage = Address | KernelAddress | Memory | Leak | Undefined |
-                     Integer | Nullability | DataFlow,
+                     Integer | Nullability | DataFlow | Fuzzer,
   RecoverableByDefault = Undefined | Integer | Nullability,
   Unrecoverable = Unreachable | Return,
   LegacyFsanitizeRecoverMask = Undefined | Integer,
   NeedsLTO = CFI,
   TrappingSupported = (Undefined & ~Vptr) | UnsignedIntegerOverflow |
                       Nullability | LocalBounds | CFI,
   TrappingDefault = CFI,
   CFIClasses = CFIVCall | CFINVCall | CFIDerivedCast | CFIUnrelatedCast,

test/Driver/fuzzer.c

 // RUN: %clang -fsanitize=fuzzer -target i386-unknown-linux -stdlib=platform %s -### 2>&1 | FileCheck --check-prefixes=CHECK-LIBCXX-LINUX %s
 //
 // CHECK-LIBCXX-LINUX: -lstdc++
 
 // RUN: %clang -target x86_64-apple-darwin14 -fsanitize=fuzzer %s -### 2>&1 | FileCheck --check-prefixes=CHECK-LIBCXX-DARWIN %s
 //
 // CHECK-LIBCXX-DARWIN: -lc++
 
+// RUN: %clang -fsanitize=fuzzer -fsanitize-coverage=trace-pc %s -### 2>&1 | FileCheck --check-prefixes=CHECK-MSG %s
+//
+// CHECK-MSG-NOT: argument unused during compilation
+
 int LLVMFuzzerTestOneInput(const char *Data, long Size) {
   return 0;
 }