Merging r246882:

------------------------------------------------------------------------
r246882 | hfinkel | 2015-09-04 17:49:21 -0400 (Fri, 04 Sep 2015) | 32 lines

Don't crash on a self-alias declaration

We were crashing in CodeGen given input like this:

  int self_alias(void) __attribute__((weak, alias("self_alias")));

such a self-alias is invalid, but instead of diagnosing the situation, we'd
proceed to produce IR for both the function declaration and the alias. Because
we already had a function named 'self_alias', the alias could not be named the
same thing, and so LLVM would pick a different name ('self_alias1' for example)
for that value. When we later called CodeGenModule::checkAliases, we'd look up
the IR value corresponding to the alias name, find the function declaration
instead, and then assert in a cast to llvm::GlobalAlias. The easiest way to prevent
this is simply to avoid creating the wrongly-named alias value in the first
place and issue the diagnostic there (instead of in checkAliases). We detect a
related cycle case in CodeGenModule::EmitAliasDefinition already, so this just
adds a second such check.

Even though the other test cases for this 'alias definition is part of a cycle'
diagnostic are in test/Sema/attr-alias-elf.c, I've added a separate regression
test for this case. This is because I can't add this check to
test/Sema/attr-alias-elf.c without disturbing the other test cases in that
file. In order to avoid construction of the bad IR values, this diagnostic
is emitted from within CodeGenModule::EmitAliasDefinition (and the relevant
declaration is not added to the Aliases vector). The other cycle checks are
done within the CodeGenModule::checkAliases function based on the Aliases
vector, called from CodeGenModule::Release.  However, if there have been errors
earlier, HandleTranslationUnit does not call Release, and so checkAliases is
never called, and so none of the other diagnostics would be produced.

Fixes PR23509.

------------------------------------------------------------------------

llvm-svn: 252808