[Backtracing] CMake fixes.

Remove a `-disable-implicit-backtracing-module-import` that was added
in `Runtimes/Core/CMakeLists.txt`, and also remove the reference to
`swiftCxxStdlib` from `RuntimeModule` as that causes the build to fail
when attempting to build the static version of `swift-backtrace` on
Linux.

rdar://124913332

Author: al45tair

Runtimes/Core/CMakeLists.txt

@@ -157,7 +157,6 @@ add_compile_options(
   "$<$<COMPILE_LANGUAGE:Swift>:SHELL:-Xfrontend -enable-experimental-concise-pound-file>"
   "$<$<COMPILE_LANGUAGE:Swift>:SHELL:-Xfrontend -enable-lexical-lifetimes=false>"
   "$<$<COMPILE_LANGUAGE:Swift>:SHELL:-Xfrontend -disable-implicit-concurrency-module-import>"
-  "$<$<COMPILE_LANGUAGE:Swift>:SHELL:-Xfrontend -disable-implicit-backtracing-module-import>"
   "$<$<COMPILE_LANGUAGE:Swift>:SHELL:-Xfrontend -disable-implicit-string-processing-module-import>"
   "$<$<COMPILE_LANGUAGE:Swift>:SHELL:-Xfrontend -enforce-exclusivity=unchecked>"
   "$<$<COMPILE_LANGUAGE:Swift>:SHELL:-Xfrontend -enable-ossa-modules>"

stdlib/public/RuntimeModule/CMakeLists.txt

@@ -14,7 +14,6 @@
 #  rather, it's a high level Swift interface to things
 set(swift_runtime_link_libraries
   swiftCore
-  swiftCxxStdlib
   swift_Concurrency
 )