[PGO] Add PGO to LDC. Supported for LLVM >= 3.7

Add the commandline options -fprofile-instr-generate[=filename] and -profile-instr-use=filename
-fprofile-instr-generate
-- Add instrumentation on branches, switches, and function entry; uses LLVM's InstrProf pass.
-- Link to profile runtime that writes instrumentation counters to a file.
-fprofile-instr-use
-- Read profile data from a file and apply branch weights to branches and switches, and annotate functions with entrycount in LLVM IR.
-- Functions with low or high entrycount are marked with 'cold' or 'inlinehint'.

The only statement type without PGO yet is "try-finally".

A new pragma, `pragma(LDC_profile_instr, [ true | false ])`, is added to selectively disable/enable instrumentation of functions (granularity = whole functions).

The runtime library ldc-profile-rt is a copy of LLVM compiler-rt lib/profile. It has to be exactly in-sync with the LLVM version, and thus we need a copy for each PGO-supported LLVM (>=3.7).
import ldc.profile for a D interface to ldc-profile-rt (for example to reset execution counts after a program startup phase).

The instrumentation data is mainly passed on to LLVM: function-entry counts and branch counts/probabilities. LDC marks functions as hot when "execution count is 30% of the maximum function execution count", and marks functions as cold if their count is 1% of maximum function execution count.

The source of LLVM's llvm-profdata tool is hereby included in LDCs repository (different source for each LLVM version), and the binary is included in the install bin folder.
The executable is named "ldc-profdata" to avoid clashing with llvm-profdata on the same machine. This is needed because profdata executable has to be in-sync with the LLVM version used to build LDC.

Maintenance burden: for trunk LLVM, we have to keep ldc-profile-rt and llvm-profdata in sync. There is no diff with upstream; but because of active development there are the occasional API changes.

driver/linker.cpp

@@ -21,6 +21,7 @@
 #include "llvm/ADT/Triple.h"
 #include "llvm/IRReader/IRReader.h"
 #include "llvm/Linker/Linker.h"
+#include "llvm/ProfileData/InstrProf.h"
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/Program.h"
 #include "llvm/Support/Path.h"
@@ -222,12 +223,28 @@ static int linkObjToBinaryGcc(bool sharedLib, bool fullyStatic) {
     args.push_back(p);
   }
 
+  // Link with profile-rt library when generating an instrumented binary
+  if (global.params.genInstrProf) {
+#if LDC_LLVM_VER >= 308
+    if (global.params.targetTriple->isOSLinux()) {
+      // For Linux, explicitly define __llvm_profile_runtime as undefined
+      // symbol, so that the initialization part of profile-rt is linked in.
+      args.push_back(("-Wl,-u," + llvm::getInstrProfRuntimeHookVarName()).str());
+    }
+#endif
+    args.push_back("-lldc-profile-rt");
+  }
+
   // default libs
   bool addSoname = false;
   switch (global.params.targetTriple->getOS()) {
   case llvm::Triple::Linux:
     addSoname = true;
-    if (!opts::disableLinkerStripDead) {
+    // Make sure we don't do --gc-sections when generating a profile-
+    // instrumented binary. The runtime relies on magic sections, which
+    // would be stripped by gc-section on older version of ld, see bug:
+    // https://sourceware.org/bugzilla/show_bug.cgi?id=19161
+    if (!opts::disableLinkerStripDead && !global.params.genInstrProf) {
       args.push_back("-Wl,--gc-sections");
     }
     if (global.params.targetTriple->getEnvironment() == llvm::Triple::Android) {
@@ -627,6 +644,11 @@ static int linkObjToBinaryWin(bool sharedLib) {
   args.push_back("comdlg32.lib");
   args.push_back("advapi32.lib");
 
+  // Link with profile-rt library when generating an instrumented binary
+  if (global.params.genInstrProf) {
+    args.push_back("ldc-profile-rt.lib");
+  }
+
   Logger::println("Linking with: ");
   Stream logstr = Logger::cout();
   for (const auto &arg : args) {