1.
In Win32NumberFormat, rename the private format() method to variadicFormat().
It's just an implementation detail of the class. Fixes:
..\..\third_party\icu\source\i18n/winnmfmt.h(146,20) :  error(clang):
        'icu_54::Win32NumberFormat::format' hides overloaded virtual functions
        [-Werror,-Woverloaded-virtual]
    UnicodeString &format(int32_t numDigits, UnicodeString &appendTo, const wchar_t *format, ...) const;
                   ^
..\..\third_party\icu\source\i18n\unicode/numfmt.h(255,28) :  note(clang):
        hidden overloaded virtual function 'icu_54::NumberFormat::format'
        declared here: different number of parameters (4 vs 3)
    virtual UnicodeString& format(const Formattable& obj,
                           ^
...

2.
In Win32DateFormat, remove the two-argument format() function that just
forwards to the base class.  (If this happens to produce compile errors,
the function should be replaced by
`using Win32DateFormat::format(UDate, UnicodeString&) const;`) Fixes:
..\..\third_party\icu\source\i18n/windtfmt.h(55,20) :  error(clang):
        'icu_54::Win32DateFormat::format' hides overloaded virtual functions
        [-Werror,-Woverloaded-virtual]
    UnicodeString& format(UDate date, UnicodeString& appendTo) const;
                   ^
..\..\third_party\icu\source\i18n\unicode/datefmt.h(240,28) :  note(clang):
        hidden overloaded virtual function 'icu_54::DateFormat::format'
        declared here: different number of parameters (4 vs 2)
    virtual UnicodeString& format(const Formattable& obj,
                           ^
...

BUG=505301
R=jshin@chromium.org

Review URL: https://codereview.chromium.org/1296803002 .