[compiler-rt] Fix reporting unrecognized flags in unittests.

[ikudrin]

Summary

With D21896, unrecognized flags might be reported within a wrong test case. For example:

[ RUN      ] SanitizerCommon.IntFlags
[       OK ] SanitizerCommon.IntFlags (9 ms)
[ RUN      ] SanitizerCommon.StrFlags
WARNING: found 2 unrecognized flag(s):
    --flag_name
    zzzzzzz
[       OK ] SanitizerCommon.StrFlags (0 ms)

Actually, these two flags are created in SanitizerCommon.IntFlags.

The patch fixes that so that the warnings are issued within the right test case:

[ RUN      ] SanitizerCommon.IntFlags
WARNING: found 1 unrecognized flag(s):
    --flag_name
WARNING: found 1 unrecognized flag(s):
    zzzzzzz
[       OK ] SanitizerCommon.IntFlags (6 ms)
[ RUN      ] SanitizerCommon.StrFlags
[       OK ] SanitizerCommon.StrFlags (0 ms)

[rnk]

lgtm, thanks!

lib/sanitizer_common/tests/sanitizer_flags_test.cc

static void TestFlag(T start_value, const char *env, T final_value) {
   T flag = start_value;
 
   FlagParser parser;
   RegisterFlag(&parser, kFlagName, kFlagDesc, &flag);
 
   parser.ParseString(env);
 
   EXPECT_EQ(final_value, flag);
+
+  // Reporting unrecognized flags is needed to reset them.
+  ReportUnrecognizedFlags();
 }
 
 template <>
 void TestFlag(const char *start_value, const char *env,
                      const char *final_value) {
   const char *flag = start_value;
 
   FlagParser parser;