[Sema][AArch64] Add parsing support for arm_sve_vector_bits attribute

Summary:

This patch implements parsing support for the 'arm_sve_vector_bits' type
attribute, defined by the Arm C Language Extensions (ACLE, version 00bet5,
section 3.7.3) for SVE [1].

The purpose of this attribute is to define fixed-length (VLST) versions
of existing sizeless types (VLAT). For example:

    #if __ARM_FEATURE_SVE_BITS==512
    typedef svint32_t fixed_svint32_t __attribute__((arm_sve_vector_bits(512)));
    #endif

Creates a type 'fixed_svint32_t' that is a fixed-length version of
'svint32_t' that is normal-sized (rather than sizeless) and contains
exactly 512 bits. Unlike 'svint32_t', this type can be used in places
such as structs and arrays where sizeless types can't.

Implemented in this patch is the following:

  * Defined and tested attribute taking single argument.
  * Checks the argument is an integer constant expression.
  * Attribute can only be attached to a single SVE vector or predicate
    type, excluding tuple types such as svint32x4_t.
  * Added the `-msve-vector-bits=<bits>` flag. When specified the
    `__ARM_FEATURE_SVE_BITS__EXPERIMENTAL` macro is defined.
  * Added a language option to store the vector size specified by the
    `-msve-vector-bits=<bits>` flag. This is used to validate `N ==
    __ARM_FEATURE_SVE_BITS`, where N is the number of bits passed to the
    attribute and `__ARM_FEATURE_SVE_BITS` is the feature macro defined under
    the same flag.

The `__ARM_FEATURE_SVE_BITS` macro will be made non-experimental in the final
patch of the series.

[1] https://developer.arm.com/documentation/100987/latest

This is patch 1/4 of a patch series.

Reviewers: sdesmalen, rsandifo-arm, efriedma, ctetreau, cameron.mcinally, rengolin, aaron.ballman

Reviewed By: sdesmalen, aaron.ballman

Differential Revision: https://reviews.llvm.org/D83550