Some ArmPL vector math routines that operate on fixed-width
vector types obey the aarch64_vector_pcs calling convention,
e.g. armpl_vsinq_f64, etc. This patch adds support for
functions in the TLI to specify an optional calling
convention to be added to the declared function. Once this
patch lands I intend to follow up with a vectoriser patch
that adds this optional calling convention when invoking the
vector library variant from the loop vectoriser.

Identified with misc-include-cleaner.

This fixes #71892 allowing us to check magled names in the IR verifier.

When creating a declaration for a vector variant, in order to determine
the argument types we need to consult the VFABI demangler. This will
allow us to add TLI mappings with linear arguments (see #76060).

A guard for empty vector function name was removed in #66656. This patch
adds the guard back.

Currently the mappings from TLI are used to generate the list of
available "scalar to vector" mappings attached to scalar calls as
"vector-function-abi-variant" LLVM IR attribute. Function names from TLI
are wrapped in mangled name following the pattern:
_ZGV<isa><mask><vlen><parameters>_<scalar_name>[(<vector_redirection>)]
The problem is the mangled name uses _LLVM_ as the ISA name which
prevents the compiler to compute vectorization factor for scalable
vectors as it cannot make any decision based on the _LLVM_ ISA. If we
use "s" as the ISA name, the compiler can make decisions based on VFABI
specification where SVE spacific rules are described.

This patch is only a refactoring stage where there is no change to the
compiler's behaviour.

This remove a bunch of #include statements in Scalar.cpp. I do not
think those should be needed any longer (assuming that they once
upon a time possibly were needed for legacy PM C bindings, but
that is not supported any longer).

Also removing some other #include statements not needed any longer
due to deprecation of legacy PM.

Differential Revision: https://reviews.llvm.org/D149438

Last user of DemandedBitsWrapperPass was the BDCE pass. Since
the legacy PM version of BDCE was removed in an earlier commit, this
patch removes the now unused DemandedBitsWrapperPass.

Differential Revision: https://reviews.llvm.org/D148336