[clang-cl] Fix value of __FUNCTION__ and __FUNC__ in MSVC mode for c++. (#66120)

Predefined macro FUNCTION (and __FUNC__) in clang is not returning the
same string than MS for templated functions.

See https://godbolt.org/z/88n1rGs3b



For this test case MSVC is returning:
function: TestClass<class UnitTestNative>::TestClass
func: TestClass

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Co-authored-by: Reid Kleckner <rnk@google.com>