Small tweaks to satisfy strict GCC/Clang warnings

Specifically, the following flags are no longer needed when building
within Chromium:

-Wno-header-guard
  Originally needed due to a typo in MinisatCore_prop.h
-Wno-mismatched-tags
  MultiplicationStats is a struct, but was forward declared as a class
-Wno-overloaded-virtual
  NonAtomIterator::ok(const ASTNode&) presumably meant to overload
  NodeIterator::ok(const ASTNode)
-Wno-parentheses-equality
  GCC/Clang suggest using "if (x == y)" and "if ((x = y))", and warn
  against "if (x = y)" and "if ((x == y))" as typos.
-Wno-return-type
  Some functions didn't always return a value;
  notably some functions didn't return because they called FatalError(),
  but the compiler didn't know that FatalError() doesn't return.
-Wno-string-conversion
  Clang doesn't like the string-literal-to-bool conversion in assert(!"foo");
  LLVM style is to write assert(0 && "foo").
-Wno-unused-const-variable
-Wno-unused-private-field
  A handful of debug variables that are never tested.