Fix sanitizer fallout from r243642

The memory-sanitizer build bot reported:

==5574== WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 0x7f03089e15f6 in
clang::Parser::TryAltiVecTokenOutOfLine(clang::DeclSpec&,
clang::SourceLocation, char const*&, unsigned int&, bool&)
/mnt/b/sanitizer-buildbot3/sanitizer-x86_64-linux-fast/build/llvm/tools/clang/lib/Parse/ParseDecl.cpp:6290:11

This is because the "Ident_pixel" variable was uninitialized
in the getLangOpts().ZVector case, but we'd still call into
clang::Parser::TryAltiVecTokenOutOfLine, which uses the variable.

The simplest fix for this without sprinkling !getLangOpts().ZVector
checks all over the code seems to be to just initialize the variable
to nullptr; this will then do the right thing on ZVector.

Checked in to unbreak the build bots.

llvm-svn: 243644