[ELF] Implement action for OUTPUT linker script command

[davide]

Summary

Parsing is in D7326

[ruiu]

LGTM

[shankarke]

LGTM.

lib/Driver/GnuLdDriver.cpp

   if (!script)
     return LinkerScriptReaderError::parse_error;
   // Evaluate script commands.
-  // Currently we only recognize this subset of linker script commands:
-  // - GROUP()
-  // - SEARCH_DIR()
+  // Currently we only recognize this subset of linker script commands.
   for (const script::Command *c : script->_commands) {
     if (auto *group = dyn_cast<script::Group>(c))
       if (std::error_code ec = evaluateLinkerScriptGroup(ctx, path, group, diag))

       ctx.addSearchPath(searchDir->getSearchPath());
     if (auto *entry = dyn_cast<script::Entry>(c))
       ctx.setEntrySymbolName(entry->getEntryName());
+    if (auto *output = dyn_cast<script::Output>(c))
+      ctx.setOutputPath(output->getOutputFileName());
   }
   return std::error_code();
 }

unittests/DriverTests/GnuLdDriverTest.cpp

   StringRef entrySymbol = _context->entrySymbolName();
   EXPECT_EQ("blah", entrySymbol);
 }
+
+TEST_F(GnuLdParserTest, LinkerScriptOutput) {
+  parse("ld", "a.o", nullptr);
+  std::unique_ptr<MemoryBuffer> mb = MemoryBuffer::getMemBuffer(
+    "OUTPUT(\"/path/to/output\")", "foo.so");
+  std::string s;
+  raw_string_ostream out(s);
+  std::error_code ec = GnuLdDriver::evalLinkerScript(
+    *_context, std::move(mb), out);
+  EXPECT_FALSE(ec);
+  StringRef output = _context->outputPath();
+  EXPECT_EQ("/path/to/output", output);
+}