Reland D78837 [lld] Remove special cases from default ld driver mode.

Drops the behavior from rL217112.

Use the Gnu driver mode by default for all platforms when ld is
invoked. Other names for the program (such as link or ld64) continue
working as before.

Reviewed By: MaskRay, srhines, smeenai, ruiu

Differential Revision: https://reviews.llvm.org/D78837

lld/test/mach-o/Inputs/linker-as-ld.yaml

@@ -1,6 +0,0 @@
---- !mach-o
-arch:            x86_64
-file-type:       MH_DYLIB
-install-name:    /usr/lib/libSystem.B.dylib
-exports:
-  - name:            dyld_stub_binder

lld/test/mach-o/linker-as-ld.yaml

@@ -1,32 +0,0 @@
-# REQUIRES: system-linker-mach-o
-#
-# RUN: rm -rf %T/ld && ln -s `which lld` %T/ld \
-# RUN:  && %T/ld -arch x86_64 -macosx_version_min 10.8 %s \
-# RUN: %p/Inputs/linker-as-ld.yaml -o %t \
-# RUN:  && llvm-nm %t | FileCheck %s
-#
-# Test linker run as "ld" on darwin works as darwin linker.
-#
-
---- !mach-o
-arch:            x86_64
-file-type:       MH_OBJECT
-flags:           [ MH_SUBSECTIONS_VIA_SYMBOLS ]
-has-UUID:        false
-OS:              unknown
-sections:
-  - segment:         __TEXT
-    section:         __text
-    type:            S_REGULAR
-    attributes:      [ S_ATTR_PURE_INSTRUCTIONS, S_ATTR_SOME_INSTRUCTIONS ]
-    address:         0x0000000000000000
-    content:         [ 0xC3 ]
-global-symbols:
-  - name:            _main
-    type:            N_SECT
-    scope:           [ N_EXT ]
-    sect:            1
-    value:           0x0000000000000000
-...
-
-# CHECK:	          T _main

lld/tools/lld/lld.cpp

@@ -96,17 +96,9 @@ static bool isPETarget(std::vector<const char *> &v) {
 }
 
 static Flavor parseProgname(StringRef progname) {
-#if __APPLE__
-  // Use Darwin driver for "ld" on Darwin.
-  if (progname == "ld")
-    return Darwin;
-#endif
-
-#if LLVM_ON_UNIX
-  // Use GNU driver for "ld" on other Unix-like system.
+  // Use GNU driver for "ld" by default.
   if (progname == "ld")
     return Gnu;
-#endif
 
   // Progname may be something like "lld-gnu". Parse it.
   SmallVector<StringRef, 3> v;