[Driver] Infer Android sysroot location.

[danalbert]

Summary

Android toolchains include their headers and libraries in a
self-contained directory within the toolchain.

[danalbert]

Ping?

[danalbert]

Ping.

[danalbert]

Ping.

[srhines]

Thanks for adding this simplified support. Sorry about the extreme delay in getting these reviewed.

lib/Driver/ToolChains/Linux.cpp

 Tool *Linux::buildAssembler() const {
   return new tools::gnutools::Assembler(*this);
 }
 
 std::string Linux::computeSysRoot() const {
   if (!getDriver().SysRoot.empty())
     return getDriver().SysRoot;
 
+  if (getTriple().isAndroid()) {
+    // Android toolchains typically include a sysroot at ../sysroot relative to
+    // the clang binary.
+    const StringRef ClangDir = getDriver().getInstalledDir();
+    std::string AndroidSysRootPath = (ClangDir + "/../sysroot").str();
+    if (getVFS().exists(AndroidSysRootPath))
+      return AndroidSysRootPath;
+  }
+
   if (!GCCInstallation.isValid() || !tools::isMipsArch(getTriple().getArch()))
     return std::string();
 
   // Standalone MIPS toolchains use different names for sysroot folder
   // and put it into different places. Here we try to check some known
   // variants.
 
   const StringRef InstallDir = GCCInstallation.getInstallPath();