[flang] Fix build failures caused by bogus GNU C++ warnings

When I do out-of-tree builds in fir-dev, I get warnings when building
lib/Parser/parsing.cpp that something may be uninitialized.  I don't get
these warnings on in-tree builds or on any builds from llvm-project.

I Googled about this and it looks like it's a bug in the GNU C++
compiler.  Adding the "-Wno-maybe-unitialized" option fixes this and
doesn't break anything else.

Author: psteinfeld

flang/CMakeLists.txt

@@ -328,6 +328,7 @@ if (FLANG_ENABLE_WERROR)
   if ( LLVM_COMPILER_IS_GCC_COMPATIBLE )
     append("-Werror" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
     append("-Wno-error" CMAKE_REQUIRED_FLAGS)
+    append("-Wno-maybe-uninitialized" CMAKE_CXX_FLAGS)
   endif( LLVM_COMPILER_IS_GCC_COMPATIBLE )
   if (NOT LLVM_ENABLE_WERROR)
       message(WARNING "FLANG_ENABLE_WERROR setting is different from LLVM_ENABLE_WERROR.")