[lldb] Fix detecting warning options for GCC

If testing for a warning option like -Wno-<foo> with GCC, GCC won't
print any diagnostic at all, leading to the options being accepted
incorrectly. However later, if compiling a file that actually prints
another warning, GCC will also print warnings about these -Wno-<foo>
options being unrecognized.

This avoids warning spam like this, for every lldb source file that
produces build warnings with GCC:

    At global scope:
    cc1plus: warning: unrecognized command line option ‘-Wno-vla-extension’
    cc1plus: warning: unrecognized command line option ‘-Wno-deprecated-register’

This matches how such warning options are detected and added in
llvm/cmake/modules/HandleLLVMOptions.cmake, e.g. like this:

    check_cxx_compiler_flag("-Wclass-memaccess" CXX_SUPPORTS_CLASS_MEMACCESS_FLAG)
    append_if(CXX_SUPPORTS_CLASS_MEMACCESS_FLAG "-Wno-class-memaccess" CMAKE_CXX_FLAGS)

Differential Revision: https://reviews.llvm.org/D123202

Author: mstorsjo

lldb/cmake/modules/LLDBConfig.cmake

@@ -168,22 +168,27 @@ else ()
 endif ()
 include_directories("${CMAKE_CURRENT_BINARY_DIR}/../clang/include")
 
+# GCC silently accepts any -Wno-<foo> option, but warns about those options
+# being unrecognized only if the compilation triggers other warnings to be
+# printed. Therefore, check for whether the compiler supports options in the
+# form -W<foo>, and if supported, add the corresponding -Wno-<foo> option.
+
 # Disable GCC warnings
-check_cxx_compiler_flag("-Wno-deprecated-declarations" CXX_SUPPORTS_NO_DEPRECATED_DECLARATIONS)
-append_if(CXX_SUPPORTS_NO_DEPRECATED_DECLARATIONS "-Wno-deprecated-declarations" CMAKE_CXX_FLAGS)
+check_cxx_compiler_flag("-Wdeprecated-declarations" CXX_SUPPORTS_DEPRECATED_DECLARATIONS)
+append_if(CXX_SUPPORTS_DEPRECATED_DECLARATIONS "-Wno-deprecated-declarations" CMAKE_CXX_FLAGS)
 
-check_cxx_compiler_flag("-Wno-unknown-pragmas" CXX_SUPPORTS_NO_UNKNOWN_PRAGMAS)
-append_if(CXX_SUPPORTS_NO_UNKNOWN_PRAGMAS "-Wno-unknown-pragmas" CMAKE_CXX_FLAGS)
+check_cxx_compiler_flag("-Wunknown-pragmas" CXX_SUPPORTS_UNKNOWN_PRAGMAS)
+append_if(CXX_SUPPORTS_UNKNOWN_PRAGMAS "-Wno-unknown-pragmas" CMAKE_CXX_FLAGS)
 
-check_cxx_compiler_flag("-Wno-strict-aliasing" CXX_SUPPORTS_NO_STRICT_ALIASING)
-append_if(CXX_SUPPORTS_NO_STRICT_ALIASING "-Wno-strict-aliasing" CMAKE_CXX_FLAGS)
+check_cxx_compiler_flag("-Wstrict-aliasing" CXX_SUPPORTS_STRICT_ALIASING)
+append_if(CXX_SUPPORTS_STRICT_ALIASING "-Wno-strict-aliasing" CMAKE_CXX_FLAGS)
 
 # Disable Clang warnings
-check_cxx_compiler_flag("-Wno-deprecated-register" CXX_SUPPORTS_NO_DEPRECATED_REGISTER)
-append_if(CXX_SUPPORTS_NO_DEPRECATED_REGISTER "-Wno-deprecated-register" CMAKE_CXX_FLAGS)
+check_cxx_compiler_flag("-Wdeprecated-register" CXX_SUPPORTS_DEPRECATED_REGISTER)
+append_if(CXX_SUPPORTS_DEPRECATED_REGISTER "-Wno-deprecated-register" CMAKE_CXX_FLAGS)
 
-check_cxx_compiler_flag("-Wno-vla-extension" CXX_SUPPORTS_NO_VLA_EXTENSION)
-append_if(CXX_SUPPORTS_NO_VLA_EXTENSION "-Wno-vla-extension" CMAKE_CXX_FLAGS)
+check_cxx_compiler_flag("-Wvla-extension" CXX_SUPPORTS_VLA_EXTENSION)
+append_if(CXX_SUPPORTS_VLA_EXTENSION "-Wno-vla-extension" CMAKE_CXX_FLAGS)
 
 # Disable MSVC warnings
 if( MSVC )