Clang 12 fails to build Flang with -DWITH_WERROR=ON

[bryanpkc]

Setting the CMake variable WITH_WERROR=ON tells Clang to compile Flang code with -Werror, which turns compile-time warnings into fatal errors. This had to be turned off by default in b8b4914 since LLVM 12 now produces many more warnings than previous LLVM versions.

I will submit a number of PRs to fix the Flang code base, in order to reduce the number of compile-time warnings, and enable WITH_WERROR=ON to be used again. Some problems requiring more extensive changes will be considered in future issues.

[bryanpkc]

List of PRs in preferred merge order (more to come):

• Fix switch- and case-related warnings; NFCI #1066
• [flang2] Fix fallthrough in mk_cmp (dinit.cpp) #1067
• [flang1] Fix malformed switch statement in dinit.c #1068
• [flang2] Delete unused empty ll_dbgutl.cpp; NFC #1069
• [flang2] Remove unused functions from mwd.cpp; NFC #1070
• [flang1][flang2] Guard unused code with macros; NFCI #1071
• [flang1] Fix -Wparentheses warnings; NFC #1072
• [flang1][flang2] Work around warnings with diagnostic pragmas; NFCI #1073
• [accpp] Prevent delete() from writing to its argument #1074
• [flang2] Delete dead code causing compile-time warnings; NFCI #1075
• Fix -Wsign-compare warnings #1076
• [runtime] Fix OPEN and UNPACKC call delegation #1077
• [runtime] Fix PRESENTC delegation #1078
• [runtime] Declare NORM2 functions in red_norm2.c; NFCI #1079
• [runtime] Declare DEALLOC03A in f90alloc.h; NFCI #1080
• [runtime] Declare MATMUL functions in matmul.h; NFCI #1081
• [scutil] Fix a couple of minor compilation warnings; NFCI #1092
• [scutil] Delete tmpf and use tmpfile instead; NFC #1126
• [CCFF] Use const char * in CCFF APIs; NFCI #1130
• [ArgParser] Use const char * for command-line arguments #1123
• [flang1][flang2] Use const char * in file generation tools; NFCI #1124
• Fix -Wunused-variable, -Wunused-function, and -Wunused-label warnings; NFCI #1160
• Fix -Wmissing-field-initializers and -Wmissing-braces warnings; NFCI #1162
• Fix const pointer-related warnings; NFCI #1164
• Fix uninitialized variable-related warnings; NFCI #1174
• Fix implicit function declaration warnings; NFCI #1180
• Fix code format-related warnings; NFC #1191

The following are "roll-up" patches addressing multiple warnings in each major component, and depend on the patches above getting merged first:

• [flang2] Fix compilation warnings; NFCI #1085
• [flang1] Fix compilation warnings; NFCI #1086
• [runtime] Fix compilation warnings; NFCI #1087