Optimize Declarator to avoid malloc/free traffic for the argument list of a

Optimize Declarator to avoid malloc/free traffic for the argument list of a
function DeclaratorChunk in common cases.  This uses a fixed array in
Declarator when it is small enough for the first function declarator chunk
in a declarator.

This eliminates all malloc/free traffic from DeclaratorChunk::getFunction
when running on Cocoa.h except for five functions: signal/bsd_signal/sigset,
which have multiple Function DeclChunk's, and
CFUUIDCreateWithBytes/CFUUIDGetConstantUUIDWithBytes, which take more than
16 arguments.

This patch was pair programmed with Steve.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@62599 91177308-0d34-0410-b5e6-96231b3b80d8