[build-script] Always set -DNDEBUG when overriding CMAKE_{C,CXX}_FLAGS_RELWITHDEBINFO for swift and llvm.

Turns out -DLLVM_ENABLE_ASSERTIONS does not actually enable assertions in LLVM!
What it actually does is rewrite the -DNDEBUG flag to be -UNDEBUG. This means
that we always want to pass in -DNDEBUG to swift. There are some complications
though. This means that we can not compile the C++ part of the runtime with a
different assertions setting than the swift compiler/llvm. This is a trade off
that for now we are ok with to get the no-asserts build working.

As part of a separate change, we are going to make -DLLVM_ENABLE_ASSERTIONS more
robust by doing what swift does now, putting -DNDEBUG or -UNDEBUG after CMake's
-DNDEBUG so we get the correct behavior of disabling or enabling assertions.

rdar://28485525

Author: gottesmm

utils/build-script-impl

@@ -1934,8 +1934,8 @@ for host in "${ALL_HOSTS[@]}"; do
                     "${cmake_options[@]}"
                     -DCMAKE_C_FLAGS="$(llvm_c_flags ${host})"
                     -DCMAKE_CXX_FLAGS="$(llvm_c_flags ${host})"
-                    -DCMAKE_C_FLAGS_RELWITHDEBINFO="-O2"
-                    -DCMAKE_CXX_FLAGS_RELWITHDEBINFO="-O2"
+                    -DCMAKE_C_FLAGS_RELWITHDEBINFO="-O2 -DNDEBUG"
+                    -DCMAKE_CXX_FLAGS_RELWITHDEBINFO="-O2 -DNDEBUG"
                     -DCMAKE_BUILD_TYPE:STRING="${LLVM_BUILD_TYPE}"
                     -DLLVM_TOOL_SWIFT_BUILD:BOOL=NO
                     -DLLVM_INCLUDE_DOCS:BOOL=TRUE
@@ -2049,8 +2049,8 @@ for host in "${ALL_HOSTS[@]}"; do
                     "${cmake_options[@]}"
                     -DCMAKE_C_FLAGS="$(swift_c_flags ${host})"
                     -DCMAKE_CXX_FLAGS="$(swift_c_flags ${host})"
-                    -DCMAKE_C_FLAGS_RELWITHDEBINFO="-O2"
-                    -DCMAKE_CXX_FLAGS_RELWITHDEBINFO="-O2"
+                    -DCMAKE_C_FLAGS_RELWITHDEBINFO="-O2 -DNDEBUG"
+                    -DCMAKE_CXX_FLAGS_RELWITHDEBINFO="-O2 -DNDEBUG"
                     -DCMAKE_BUILD_TYPE:STRING="${SWIFT_BUILD_TYPE}"
                     -DLLVM_ENABLE_ASSERTIONS:BOOL=$(true_false "${SWIFT_ENABLE_ASSERTIONS}")
                     -DSWIFT_ANALYZE_CODE_COVERAGE:STRING=$(toupper "${SWIFT_ANALYZE_CODE_COVERAGE}")