Add a new "available_externally" linkage type. This is intended

to support C99 inline, GNU extern inline, etc.  Related bugzilla's
include PR3517, PR3100, & PR2933.  Nothing uses this yet, but it
appears to work.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@68940 91177308-0d34-0410-b5e6-96231b3b80d8

test/Transforms/GlobalDCE/externally_available.ll

@@ -0,0 +1,10 @@
+; RUN: llvm-as < %s | opt -globaldce | llvm-dis | not grep test_
+
+; test_function should not be emitted to the .s file.
+define available_externally i32 @test_function() {
+  ret i32 4
+}
+
+; test_global should not be emitted to the .s file.
+@test_global = available_externally global i32 4
+