Fix LLVM global variable linkage
Fix LLVM global variable linkage

[reviewed by Kyle]

The LLVM backend, like the C backend, has supported compilation-unit local
functions (ie 'static' for any not-exported proc). This patch adds that
functionality to global variables, which previously were always code generated
with external linkage.

Verified with CHPL_LLVM=none make check; verified symbols had appropriate types
with 'nm' for --llvm; verified Hello World works with --llvm, verified
full local test suite with --llvm.

(Note that currently 'export var x:int' fails with a syntax error;
   the way I tested exporting a variable was with pragma "export".)



git-svn-id: 88467cb1fb04b8a755be7e1ee1026be4190196ef@23019 3a8e244f-b0f2-452b-bcba-4c88e055c3ca

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