[build-script] Add flag to export compile commands

CMake supports generation of compilation databases. These have many uses, not
least of which is in supporting code-completion engines like
https://github.com/Valloric/YouCompleteMe.

Add the `--export-compile-commands` flag to `utils/build-script`.
Setting this flag passes `-DCMAKE_EXPORT_COMPILE_COMMANDS=ON` to CMake,
which results in a `compile_commands.json` file being generated along
with built products like `cmark`, `llvm`, and `swift`.

utils/build-script-impl

@@ -182,7 +182,7 @@ KNOWN_SETTINGS=(
     darwin-toolchain-installer-package ""        "The path to installer pkg"
     build-jobs ""                                "The number of parallel build jobs to use"
     darwin-toolchain-alias             "swift"   "Swift alias for toolchain"
-
+    export-compile-commands            ""        "set to generate JSON compilation databases for each build product"
 )
 
 function toupper() {
@@ -1035,6 +1035,13 @@ if [[ "${ENABLE_ASAN}" ]] ; then
     )
 fi
 
+if [[ "${EXPORT_COMPILE_COMMANDS}" ]] ; then
+    COMMON_CMAKE_OPTIONS=(
+        "${COMMON_CMAKE_OPTIONS[@]}"
+        -DCMAKE_EXPORT_COMPILE_COMMANDS=ON
+    )
+fi
+
 if [ -z "${HOST_CC}" ] ; then
     if [ "$(uname -s)" == "Darwin" ] ; then
         HOST_CC="$(xcrun_find_tool clang)"