[SR-8018] llbuild fails to build with SwiftPM because of the DEBUG define

[hartbit]

Previous ID	SR-8018
Radar	None
Original Reporter	@hartbit
Type	Bug

Additional Detail from JIRA

Votes	2
Component/s	llbuild
Labels	Bug
Assignee	None
Priority	Medium

md5: db359f7c39800a3e55bf94a0e124261c

Issue Description:

When building llbuild with SwiftPM, we currently get the following error:

In file included from /Users/david/Projects/swift/llbuild/lib/llvm/Support/Path.cpp:14:
/Users/david/Projects/swift/llbuild/lib/llvm/Support/include/llvm/Support/COFF.h:541:5: error: expected identifier
    DEBUG,
    ^
<command line>:2:15: note: expanded from here
#define DEBUG 1
              ^
1 error generated.

This seems to be because clang is invoked with a -DDEBUG=1 define that clashes with llvmSupport's DataDirectoryIndex enum:

enum DataDirectoryIndex {
  EXPORT_TABLE = 0,
  IMPORT_TABLE,
  RESOURCE_TABLE,
  EXCEPTION_TABLE,
  CERTIFICATE_TABLE,
  BASE_RELOCATION_TABLE,
  DEBUG, // HERE
  ARCHITECTURE,
  GLOBAL_PTR,
  TLS_TABLE,
  LOAD_CONFIG_TABLE,
  BOUND_IMPORT,
  IAT,
  DELAY_IMPORT_DESCRIPTOR,
  CLR_RUNTIME_HEADER,

  NUM_DATA_DIRECTORIES
};

The only solution I've found for now is to tell SwiftPM to undefine DEBUG:

swift build -Xcxx -UDEBUG