[flang] Disable -Wmaybe-uninitialized for GCC

[tskeith]

Summary

According to llvm/cmake/config-ix.cmake, gcc's potentially uninitialized
use analysis has lots of false positives. We are encountering one in
flang/lib/Lower/CharacterExpr.cpp.

That warning is disabled for gcc in in-tree builds; this does the same
thing for out-of-tree builds.

[PeteSteinfeld]

Everything built and tested successfully in GCC both in-tree and out-of-tree.

flang/CMakeLists.txt

if (CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
   list(APPEND CMAKE_MODULE_PATH ${LLVM_DIR})
 
   # If LLVM links to zlib we need the imported targets so we can too.
   if(LLVM_ENABLE_ZLIB)
     find_package(ZLIB REQUIRED)
   endif()
   option(LLVM_ENABLE_WARNINGS "Enable compiler warnings." ON)
   option(LLVM_ENABLE_PEDANTIC "Compile with pedantic enabled." ON)
+  if(CMAKE_COMPILER_IS_GNUCXX)
+    set(USE_NO_MAYBE_UNINITIALIZED 1)
+  endif()
 
   include(CMakeParseArguments)
   include(AddLLVM)
   include(HandleLLVMOptions)
   include(VersionFromVCS)
 
   if(LINK_WITH_FIR)
     include(TableGen)