This patch continues simplifying `ParseChildParameters` by moving out
the logic that parses the first parameter of a function DIE into a
helper function. Since with GCC (and lately Clang) function declarations
have `DW_AT_object_pointer`s, we should be able to check for the
attribute's existence to determine if a function is static (and also
deduce CV-qualifiers from it). This will be useful for cases where the
object parameter is explicit (which is possible since C++23).
This should be NFC. I added a FIXME to places where we assume an
implicit object parameter (which will be addressed in a follow-up
patch).
We used to guard parsing of the CV-qualifiers of the "this" parameter
with a `encoding_mask & Type::eEncodingIsPointerUID`, which is
incorrect, because `eEncodingIsPointerUID` cannot be used as a bitmask
directly (see https://github.com/llvm/llvm-project/issues/120856). This
patch corrects this, but it should still be NFC because any parameter in
C++ called "this" *is* an implicit object parameter.
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