Add comments explaining the different profile files.

Also adjust ifdef to be a bit more clear.

Author: efriedma-quic

compiler-rt/lib/profile/InstrProfilingPlatformLinux.c

@@ -6,16 +6,30 @@
 |*
 \*===----------------------------------------------------------------------===*/
 
+// This file defines profile data symbols for ELF, wasm, XCOFF. It assumes
+// __start_ and __stop_ symbols for profile data point at the beginning and
+// end of the sections in question.  (This is technically a linker feature,
+// not a file format feature, but linkers for these targets support it.)
+//
+// MachO (MacOS/iOS) and PE-COFF (Windows) have a similar support, but the
+// identifiers are different, so the support is in separate files.
+//
+// Support for targets which don't have linker support is in
+// InstrProfilingPlatformOther.c.
+//
+// This file also contains code to extract ELF build IDs from the ELF file,
+// to identify the build which generated the file.
+
 #if defined(__linux__) || defined(__FreeBSD__) || defined(__Fuchsia__) ||      \
     (defined(__sun__) && defined(__svr4__)) || defined(__NetBSD__) ||          \
     defined(_AIX) || defined(__wasm__) || defined(__HAIKU__) ||                \
     defined(COMPILER_RT_PROFILE_BAREMETAL)
 
-#ifndef COMPILER_RT_PROFILE_BAREMETAL
-#if !defined(_AIX) && !defined(__wasm__)
+#if !defined(_AIX) && !defined(__wasm__) &&                                    \
+    !defined(COMPILER_RT_PROFILE_BAREMETAL)
+// Includes for non-baremetal ELF targets, used to output build IDs.
 #include <elf.h>
 #include <link.h>
-#endif
 #include <stdlib.h>
 #include <string.h>
 #endif

compiler-rt/lib/profile/InstrProfilingPlatformOther.c

@@ -6,6 +6,13 @@
 |*
 \*===----------------------------------------------------------------------===*/
 
+// This file defines a fallback implementation to compute the locations of
+// profile data sections, for targets that don't have linker support.  No
+// commonly used targets use this codepath.
+//
+// This implementation expects the compiler instrumentation pass to define a
+// constructor in each file which calls into this file.
+
 #if !defined(__APPLE__) && !defined(__linux__) && !defined(__FreeBSD__) &&     \
     !defined(__Fuchsia__) && !(defined(__sun__) && defined(__svr4__)) &&       \
     !defined(__NetBSD__) && !defined(_WIN32) && !defined(_AIX) &&              \