[Toolchain] Lookup clang in the toolchain

Now that we have clang shipping in the Swift toolchains, we can just
locate it in the bin dir.

<rdar://problem/51077480>
https://bugs.swift.org/browse/SR-10633

Author: aciidgh

Sources/Workspace/UserToolchain.swift

@@ -151,30 +151,35 @@ public final class UserToolchain: Toolchain {
             return toolPath
         }
 
-        // Otherwise, lookup the tool on the system.
+        // Then, check the toolchain.
+        do {
+            let toolPath = destination.binDir.appending(component: "clang" + hostExecutableSuffix)
+            if localFileSystem.exists(toolPath) {
+                _clangCompiler = toolPath
+                return toolPath
+            }
+        }
+
+        // Otherwise, lookup it up on the system.
         let arguments = whichArgs + ["clang"]
         let foundPath = try Process.checkNonZeroExit(arguments: arguments, environment: processEnvironment).spm_chomp()
         guard !foundPath.isEmpty else {
             throw InvalidToolchainDiagnostic("could not find clang")
         }
         let toolPath = try AbsolutePath(validating: foundPath)
-
-        // If we found clang using xcrun, assume the vendor is Apple.
-        // FIXME: This might not be the best way to determine this.
-        #if os(macOS)
-            __isClangCompilerVendorApple = true
-        #endif
-
         _clangCompiler = toolPath
         return toolPath
     }
     private var _clangCompiler: AbsolutePath?
-    private var __isClangCompilerVendorApple: Bool?
 
     public func _isClangCompilerVendorApple() throws -> Bool? {
-        // The boolean gets computed as a side-effect of lookup for clang compiler.
-        _ = try getClangCompiler()
-        return __isClangCompilerVendorApple
+        // Assume the vendor is Apple on macOS.
+        // FIXME: This might not be the best way to determine this.
+      #if os(macOS)
+        return true
+      #else
+        return false
+      #endif
     }
 
     /// Returns the path to llvm-cov tool.

Utilities/bootstrap

@@ -306,7 +306,7 @@ class Target(object):
 
             link_input_nodes.append(object_path)
 
-            args = ["clang"]
+            args = [args.clang_path]
             args.extend(common_args)
             args.extend([deps_path, "-c", source,"-o", object_path])
 
@@ -733,6 +733,20 @@ def get_llbuild_source_path():
     note("clone llbuild next to swiftpm directory; see development docs: https://github.com/apple/swift-package-manager/blob/master/Documentation/Development.md#using-trunk-snapshot")
     error("unable to find llbuild source directory at %s" % llbuild_path)
 
+def get_clang_path():
+    try:
+        if os.getenv("CC"):
+            clang_path=os.path.realpath(os.getenv("CC"))
+            return clang_path
+        elif platform.system() == 'Darwin':
+            return subprocess.check_output(["xcrun", "--find", "clang"],
+                stderr=subprocess.PIPE, universal_newlines=True).strip()
+        else:
+            return subprocess.check_output(["which", "clang"],
+            universal_newlines=True).strip()
+    except:
+        error("unable to find 'clang' tool for bootstrap build")
+
 def get_swiftc_path():
     try:
         if os.getenv("SWIFT_EXEC"):
@@ -978,6 +992,7 @@ def main():
 
     if not args.swiftc_path:
         args.swiftc_path = get_swiftc_path()
+    args.clang_path = get_clang_path()
 
     args.swift_stdlib_path = os.path.normpath(
         os.path.join(os.path.dirname(os.path.realpath(args.swiftc_path)), "..",
@@ -1128,6 +1143,7 @@ def main():
     def make_fake_toolchain():
         symlink_force(args.swiftc_path, os.path.join(bindir, "swift"))
         symlink_force(args.swiftc_path, os.path.join(bindir, "swiftc"))
+        symlink_force(args.clang_path, os.path.join(bindir, "clang"))
         symlink_force(args.sbt_path, os.path.join(bindir, "swift-build-tool"))
         symlink_force(os.path.join(sandbox_path, "bin", "swift-build"),
                       bootstrapped_product)