[Fuchsia] Use -ffile-prefix-map

[phosek]

Summary

This makes toolchain independent of the path it was built in by
rewriting all absolute paths embedded in sources and debug info
into relative ones.

[dblaikie]

@leonardchan sorry for the inconvenience, but Phabricator has a bug/missing feature where it doesn't send mail for bug updates that contain no user-authored text, so approving a patch without writing anything in the text box doesn't send an email to the mailing lrist (the mailing list is still considered the canonical source of truth for reviews, etc) and thus makes it looks like patches are being committed without review. When approving a patch through phab, please write something (eg: "Thanks!", "Looks good!", etc) in the text box to workaround this bug.

clang/cmake/caches/Fuchsia-stage2.cmake

 set(LLVM_ENABLE_LTO ON CACHE BOOL "")
 set(LLVM_ENABLE_PER_TARGET_RUNTIME_DIR ON CACHE BOOL "")
 set(LLVM_ENABLE_TERMINFO OFF CACHE BOOL "")
 set(LLVM_ENABLE_UNWIND_TABLES OFF CACHE BOOL "")
 set(LLVM_ENABLE_ZLIB ON CACHE BOOL "")
 set(LLVM_INCLUDE_DOCS OFF CACHE BOOL "")
 set(LLVM_INCLUDE_EXAMPLES OFF CACHE BOOL "")
 set(LLVM_INCLUDE_GO_TESTS OFF CACHE BOOL "")
-set(LLVM_USE_RELATIVE_PATHS_IN_DEBUG_INFO ON CACHE BOOL "")
+set(LLVM_USE_RELATIVE_PATHS_IN_FILES ON CACHE BOOL "")
 
 set(CLANG_DEFAULT_CXX_STDLIB libc++ CACHE STRING "")
 if(NOT APPLE)
   set(CLANG_DEFAULT_LINKER lld CACHE STRING "")
   set(CLANG_DEFAULT_OBJCOPY llvm-objcopy CACHE STRING "")
 endif()
 set(CLANG_DEFAULT_RTLIB compiler-rt CACHE STRING "")
 set(CLANG_ENABLE_ARCMT OFF CACHE BOOL "")
if(LINUX_${target}_SYSROOT)
     # Set the per-target builtins options.
     list(APPEND BUILTIN_TARGETS "${target}")
     set(BUILTINS_${target}_CMAKE_SYSTEM_NAME Linux CACHE STRING "")
     set(BUILTINS_${target}_CMAKE_BUILD_TYPE RelWithDebInfo CACHE STRING "")
     set(BUILTINS_${target}_CMAKE_SYSROOT ${LINUX_${target}_SYSROOT} CACHE STRING "")
     set(BUILTINS_${target}_CMAKE_SHARED_LINKER_FLAGS "-fuse-ld=lld" CACHE STRING "")
     set(BUILTINS_${target}_CMAKE_MODULE_LINKER_FLAGS "-fuse-ld=lld" CACHE STRING "")
     set(BUILTINS_${target}_CMAKE_EXE_LINKER_FLAG "-fuse-ld=lld" CACHE STRING "")
+    set(BUILTINS_${target}_LLVM_USE_RELATIVE_PATHS_IN_FILES ON CACHE BOOL "")
 
     # Set the per-target runtimes options.
     list(APPEND RUNTIME_TARGETS "${target}")
     set(RUNTIMES_${target}_CMAKE_SYSTEM_NAME Linux CACHE STRING "")
     set(RUNTIMES_${target}_CMAKE_BUILD_TYPE RelWithDebInfo CACHE STRING "")
     set(RUNTIMES_${target}_CMAKE_SYSROOT ${LINUX_${target}_SYSROOT} CACHE STRING "")
     set(RUNTIMES_${target}_CMAKE_SHARED_LINKER_FLAGS "-fuse-ld=lld" CACHE STRING "")
     set(RUNTIMES_${target}_CMAKE_MODULE_LINKER_FLAGS "-fuse-ld=lld" CACHE STRING "")
     set(RUNTIMES_${target}_CMAKE_EXE_LINKER_FLAGS "-fuse-ld=lld" CACHE STRING "")
     set(RUNTIMES_${target}_LLVM_ENABLE_ASSERTIONS ON CACHE BOOL "")
+    set(RUNTIMES_${target}_LLVM_USE_RELATIVE_PATHS_IN_FILES ON CACHE BOOL "")
     set(RUNTIMES_${target}_LIBUNWIND_ENABLE_SHARED OFF CACHE BOOL "")
     set(RUNTIMES_${target}_LIBUNWIND_USE_COMPILER_RT ON CACHE BOOL "")
     set(RUNTIMES_${target}_LIBUNWIND_INSTALL_LIBRARY OFF CACHE BOOL "")
     set(RUNTIMES_${target}_LIBCXXABI_USE_COMPILER_RT ON CACHE BOOL "")
     set(RUNTIMES_${target}_LIBCXXABI_ENABLE_SHARED OFF CACHE BOOL "")
     set(RUNTIMES_${target}_LIBCXXABI_USE_LLVM_UNWINDER ON CACHE BOOL "")
     set(RUNTIMES_${target}_LIBCXXABI_ENABLE_STATIC_UNWINDER ON CACHE BOOL "")
     set(RUNTIMES_${target}_LIBCXXABI_INSTALL_LIBRARY OFF CACHE BOOL "")
foreach(target x86_64;aarch64;riscv64)
     set(BUILTINS_${target}-unknown-fuchsia_CMAKE_BUILD_TYPE RelWithDebInfo CACHE STRING "")
     set(BUILTINS_${target}-unknown-fuchsia_CMAKE_ASM_FLAGS ${FUCHSIA_${target}_COMPILER_FLAGS} CACHE STRING "")
     set(BUILTINS_${target}-unknown-fuchsia_CMAKE_C_FLAGS ${FUCHSIA_${target}_COMPILER_FLAGS} CACHE STRING "")
     set(BUILTINS_${target}-unknown-fuchsia_CMAKE_CXX_FLAGS ${FUCHSIA_${target}_COMPILER_FLAGS} CACHE STRING "")
     set(BUILTINS_${target}-unknown-fuchsia_CMAKE_SHARED_LINKER_FLAGS ${FUCHSIA_${target}_LINKER_FLAGS} CACHE STRING "")
     set(BUILTINS_${target}-unknown-fuchsia_CMAKE_MODULE_LINKER_FLAGS ${FUCHSIA_${target}_LINKER_FLAGS} CACHE STRING "")
     set(BUILTINS_${target}-unknown-fuchsia_CMAKE_EXE_LINKER_FLAGS ${FUCHSIA_${target}_LINKER_FLAGS} CACHE STRING "")
     set(BUILTINS_${target}-unknown-fuchsia_CMAKE_SYSROOT ${FUCHSIA_${target}_SYSROOT} CACHE PATH "")
+    set(BUILTINS_${target}-unknown-fuchsia_LLVM_USE_RELATIVE_PATHS_IN_FILES ON CACHE BOOL "")
   endforeach()
 
   foreach(target x86_64;aarch64)
     # Set the per-target runtimes options.
     list(APPEND RUNTIME_TARGETS "${target}-unknown-fuchsia")
     set(RUNTIMES_${target}-unknown-fuchsia_CMAKE_SYSTEM_NAME Fuchsia CACHE STRING "")
     set(RUNTIMES_${target}-unknown-fuchsia_CMAKE_BUILD_TYPE RelWithDebInfo CACHE STRING "")
     set(RUNTIMES_${target}-unknown-fuchsia_CMAKE_BUILD_WITH_INSTALL_RPATH ON CACHE BOOL "")
     set(RUNTIMES_${target}-unknown-fuchsia_CMAKE_ASM_FLAGS ${FUCHSIA_${target}_COMPILER_FLAGS} CACHE STRING "")
     set(RUNTIMES_${target}-unknown-fuchsia_CMAKE_C_FLAGS ${FUCHSIA_${target}_COMPILER_FLAGS} CACHE STRING "")
     set(RUNTIMES_${target}-unknown-fuchsia_CMAKE_CXX_FLAGS ${FUCHSIA_${target}_COMPILER_FLAGS} CACHE STRING "")
     set(RUNTIMES_${target}-unknown-fuchsia_CMAKE_SHARED_LINKER_FLAGS ${FUCHSIA_${target}_LINKER_FLAGS} CACHE STRING "")
     set(RUNTIMES_${target}-unknown-fuchsia_CMAKE_MODULE_LINKER_FLAGS ${FUCHSIA_${target}_LINKER_FLAGS} CACHE STRING "")
     set(RUNTIMES_${target}-unknown-fuchsia_CMAKE_EXE_LINKER_FLAGS ${FUCHSIA_${target}_LINKER_FLAGS} CACHE STRING "")
     set(RUNTIMES_${target}-unknown-fuchsia_CMAKE_SYSROOT ${FUCHSIA_${target}_SYSROOT} CACHE PATH "")
     set(RUNTIMES_${target}-unknown-fuchsia_LLVM_ENABLE_ASSERTIONS ON CACHE BOOL "")
+    set(RUNTIMES_${target}-unknown-fuchsia_LLVM_USE_RELATIVE_PATHS_IN_FILES ON CACHE BOOL "")
     set(RUNTIMES_${target}-unknown-fuchsia_LIBUNWIND_USE_COMPILER_RT ON CACHE BOOL "")
     set(RUNTIMES_${target}-unknown-fuchsia_LIBUNWIND_HERMETIC_STATIC_LIBRARY ON CACHE BOOL "")
     set(RUNTIMES_${target}-unknown-fuchsia_LIBUNWIND_INSTALL_STATIC_LIBRARY OFF CACHE BOOL "")
     set(RUNTIMES_${target}-unknown-fuchsia_LIBCXXABI_USE_COMPILER_RT ON CACHE BOOL "")
     set(RUNTIMES_${target}-unknown-fuchsia_LIBCXXABI_USE_LLVM_UNWINDER ON CACHE BOOL "")
     set(RUNTIMES_${target}-unknown-fuchsia_LIBCXXABI_ENABLE_STATIC_UNWINDER ON CACHE BOOL "")
     set(RUNTIMES_${target}-unknown-fuchsia_LIBCXXABI_HERMETIC_STATIC_LIBRARY ON CACHE BOOL "")
     set(RUNTIMES_${target}-unknown-fuchsia_LIBCXXABI_INSTALL_STATIC_LIBRARY OFF CACHE BOOL "")