Merge bitcoin#19015: build: Enable some commonly enabled compiler dia…

…gnostics

2f8a4c9 build: Enable some commonly enabled compiler diagnostics (practicalswift)

Pull request description:

  Enable some commonly enabled compiler diagnostics as discussed in bitcoin#17344.

  | Compiler diagnostic | no# of emitted unique GCC warnings in `master` | no# of emitted unique Clang warnings in `master` |
  | ------------- | ------------- | ------------- |
  | `-Wduplicated-branches`: Warn if `if`/`else` branches have duplicated code  | 0 | Not supported |
  | `-Wduplicated-cond`: Warn if `if`/`else` chain has duplicated conditions  | 0 | Not supported |
  | `-Wlogical-op`: Warn about logical operations being used where bitwise were probably wanted  | 0 | Not supported |
  | `-Woverloaded-virtual`: Warn if you overload (not `override`) a virtual function  | 0 | 0 |
  | ~~`-Wunused-member-function`: Warn on unused member function~~  | Not supported | 2 |
  | ~~`-Wunused-template`: Warn on unused template~~ | Not supported | 1 |

  There is a large overlap between this list and [Jason Turner's list of recommended compiler diagnostics in the Collaborative Collection of C++ Best Practices (`cppbestpractices`) project](https://github.com/lefticus/cppbestpractices/blob/master/02-Use_the_Tools_Available.md#gcc--clang). There is also an overlap with the recommendations given in the [C++ Core Guidelines](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines) (with editors Bjarne Stroustrup and Herb Sutter).

  Closes bitcoin#17344.

ACKs for top commit:
  jonatack:
    ACK 2f8a4c9 no warnings for me with these locally on debian 5.7.10-1 (2020-07-26) x86_64 with gcc 10 and clang 12
  fanquake:
    ACK 2f8a4c9 - no-longer seeing any obvious issues with doing this.
  hebasto:
    ACK 2f8a4c9, no new warnings in Travis jobs.

Tree-SHA512: f669ea22b31263a555f999eff6a9d65750662e95831b188c3192a2cf0127fb7b5136deb762a6b0b7bbdfb0dc6a40caf48251a62b164fffb81dd562bdd15ec3c8

configure.ac

@@ -404,6 +404,10 @@ if test "x$CXXFLAGS_overridden" = "xno"; then
   AX_CHECK_COMPILE_FLAG([-Wdate-time],[WARN_CXXFLAGS="$WARN_CXXFLAGS -Wdate-time"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Wconditional-uninitialized],[WARN_CXXFLAGS="$WARN_CXXFLAGS -Wconditional-uninitialized"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Wsign-compare],[WARN_CXXFLAGS="$WARN_CXXFLAGS -Wsign-compare"],,[[$CXXFLAG_WERROR]])
+  AX_CHECK_COMPILE_FLAG([-Wduplicated-branches],[WARN_CXXFLAGS="$WARN_CXXFLAGS -Wduplicated-branches"],,[[$CXXFLAG_WERROR]])
+  AX_CHECK_COMPILE_FLAG([-Wduplicated-cond],[WARN_CXXFLAGS="$WARN_CXXFLAGS -Wduplicated-cond"],,[[$CXXFLAG_WERROR]])
+  AX_CHECK_COMPILE_FLAG([-Wlogical-op],[WARN_CXXFLAGS="$WARN_CXXFLAGS -Wlogical-op"],,[[$CXXFLAG_WERROR]])
+  AX_CHECK_COMPILE_FLAG([-Woverloaded-virtual],[WARN_CXXFLAGS="$WARN_CXXFLAGS -Woverloaded-virtual"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Wsuggest-override],[WARN_CXXFLAGS="$WARN_CXXFLAGS -Wsuggest-override"],,[[$CXXFLAG_WERROR]],
                         [AC_LANG_SOURCE([[struct A { virtual void f(); }; struct B : A { void f() final; };]])])
   AX_CHECK_COMPILE_FLAG([-Wunreachable-code-loop-increment],[WARN_CXXFLAGS="$WARN_CXXFLAGS -Wunreachable-code-loop-increment"],,[[$CXXFLAG_WERROR]])