[clang-cl] Fix value of __FUNCTION__ in MSVC mode. (#84014)

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

See https://godbolt.org/z/q3EKn5zq4

For the same test case MSVC is returning:

function: TestClass::TestClass
function: TestStruct::TestStruct
function: TestEnum::TestEnum

The initial work for this was in the reverted patch
(https://github.com/llvm/llvm-project/pull/66120). This patch solves the
issues raised in the reverted patch.