[build-script] Only hard code the C/CXX flags for RelWithDebInfo builds.

This patch resolves some post-commit feedback from @llvmbeanz. Specifically, the
only time that we need to hard code these flags are when compiling with
RelWithDebInfo since we need extra control in that case to ensure that CMake
does not sneak in a -g flag on the command line. This is important to ensure
that we are compiling with -gline-tables-only instead of -g.

Given that it is a performance fix specific to compiling with LTO with debug
info, we should no perform such a change when compiling with:

1. MINRELSIZE since we do not ever perform that build.
2. RELEASE since it does not have debug info.
3. DEBUG since why would one perform an LTO build in DEBUG mode.

In terms of 3, I can potentially imagine someone trying to compile Swift in
Debug with LTO to try to debug an LTO bug, but I imagine such a test case to be
such a rare need that there is no point in trying to make it faster.

Author: gottesmm

utils/build-script-impl

@@ -1934,14 +1934,8 @@ for host in "${ALL_HOSTS[@]}"; do
                     "${cmake_options[@]}"
                     -DCMAKE_C_FLAGS="$(llvm_c_flags ${host})"
                     -DCMAKE_CXX_FLAGS="$(llvm_c_flags ${host})"
-                    -DCMAKE_C_FLAGS_DEBUG=""
-                    -DCMAKE_CXX_FLAGS_DEBUG=""
-                    -DCMAKE_C_FLAGS_RELEASE="-O3"
-                    -DCMAKE_CXX_FLAGS_RELEASE="-O3"
                     -DCMAKE_C_FLAGS_RELWITHDEBINFO="-O2"
                     -DCMAKE_CXX_FLAGS_RELWITHDEBINFO="-O2"
-                    -DCMAKE_C_FLAGS_MINSIZEREL="-Os"
-                    -DCMAKE_CXX_FLAGS_MINSIZEREL="-Os"
                     -DCMAKE_BUILD_TYPE:STRING="${LLVM_BUILD_TYPE}"
                     -DLLVM_TOOL_SWIFT_BUILD:BOOL=NO
                     -DLLVM_INCLUDE_DOCS:BOOL=TRUE
@@ -2055,14 +2049,8 @@ for host in "${ALL_HOSTS[@]}"; do
                     "${cmake_options[@]}"
                     -DCMAKE_C_FLAGS="$(swift_c_flags ${host})"
                     -DCMAKE_CXX_FLAGS="$(swift_c_flags ${host})"
-                    -DCMAKE_C_FLAGS_DEBUG=""
-                    -DCMAKE_CXX_FLAGS_DEBUG=""
-                    -DCMAKE_C_FLAGS_RELEASE="-O3"
-                    -DCMAKE_CXX_FLAGS_RELEASE="-O3"
                     -DCMAKE_C_FLAGS_RELWITHDEBINFO="-O2"
                     -DCMAKE_CXX_FLAGS_RELWITHDEBINFO="-O2"
-                    -DCMAKE_C_FLAGS_MINSIZEREL="-Os"
-                    -DCMAKE_CXX_FLAGS_MINSIZEREL="-Os"
                     -DCMAKE_BUILD_TYPE:STRING="${SWIFT_BUILD_TYPE}"
                     -DLLVM_ENABLE_ASSERTIONS:BOOL=$(true_false "${SWIFT_ENABLE_ASSERTIONS}")
                     -DSWIFT_ANALYZE_CODE_COVERAGE:STRING=$(toupper "${SWIFT_ANALYZE_CODE_COVERAGE}")