Transform: add SymbolRewriter pass

This introduces the symbol rewriter. This is an IR->IR transformation that is
implemented as a CodeGenPrepare pass. This allows for the transparent
adjustment of the symbols during compilation.

It provides a clean, simple, elegant solution for symbol inter-positioning. This
technique is often used, such as in the various sanitizers and performance
analysis.

The control of this is via a custom YAML syntax map file that indicates source
to destination mapping, so as to avoid having the compiler to know the exact
details of the source to destination transformations.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221548 91177308-0d34-0410-b5e6-96231b3b80d8

tools/opt/opt.cpp

@@ -322,6 +322,7 @@ int main(int argc, char **argv) {
   // supported.
   initializeCodeGenPreparePass(Registry);
   initializeAtomicExpandPass(Registry);
+  initializeRewriteSymbolsPass(Registry);
 
 #ifdef LINK_POLLY_INTO_TOOLS
   polly::initializePollyPasses(Registry);