[AST] Move back BasePathSize to the bit-fields of CastExpr

The number of trailing CXXBaseSpecifiers in CastExpr was moved from
CastExprBitfields to a trailing object in r338489 (D50050). At this time these
bit-fields classes were only 32 bits wide. However later r345459 widened these
bit-field classes to 64 bits.

The reason for this change was that on 64 bit archs alignment requirements
caused 4 bytes of padding after the Stmt sub-object in nearly all expression
classes. Reusing this padding yielded an >10% reduction in the size used by all
statement/expressions when parsing all of Boost (on a 64 bit arch). This
increased the size of statement/expressions for 32 bits archs, but this can be
mitigated by moving more data to the bit-fields of Stmt (and moreover most
people now care about 64 bits archs as a host).

Therefore move back the number of CXXBaseSpecifiers in CastExpr to the
bit-fields of Stmt. This in effect mostly revert r338489 while keeping the
added test.

Differential Revision: https://reviews.llvm.org/D56358

Reviewed By: lebedev.ri

Reviewers: lebedev.ri, rjmccall
llvm-svn: 350741