Again, tools are trickier to pick the main module header for than
library source files. I've started to follow the pattern of using
LLVMContext.h when it is included as a stub for program source files.

llvm-svn: 169252

llvm-svn: 167757

llvm-svn: 167730

This is the second and last (2/2) part of a change that moves llvm-symbolizer to llvm/tools/, which will allow to build it
with both cmake and configure+make.

llvm-svn: 167723