[LLDB] Prefer serialized bridging headers.

Especially in an explicit modules project, LLDB might not know all the
search paths needed to imported the on disk header.

rdar://156542351

Author: adrian-prantl

lldb/test/API/lang/swift/explicit_modules/bridgingheader/Makefile

@@ -0,0 +1,14 @@
+SWIFT_SOURCES := main.swift
+SWIFT_ENABLE_EXPLICIT_MODULES := Yes
+SWIFT_BRIDGING_HEADER := bridging.h
+SWIFTFLAGS_EXTRAS = -Xcc -I$(BUILDDIR)/secret
+
+all: secret.h $(EXE)
+
+include Makefile.rules
+
+.PHONY: secret.h
+
+secret.h:
+	mkdir -p $(BUILDDIR)/secret
+	echo "struct S { int i; };" > $(BUILDDIR)/secret/secret.h

lldb/test/API/lang/swift/explicit_modules/bridgingheader/TestSwiftExplicitModulesBridgingHeader.py

@@ -0,0 +1,26 @@
+import lldb
+from lldbsuite.test.decorators import *
+import lldbsuite.test.lldbtest as lldbtest
+import lldbsuite.test.lldbutil as lldbutil
+
+
+class TestSwiftExplicitModules(lldbtest.TestBase):
+    NO_DEBUG_INFO_TESTCASE = True
+    @swiftTest
+    def test(self):
+        """Test explicit Swift modules with bridging headers"""
+        self.build()
+        secret = self.getBuildArtifact("secret")
+        import shutil
+        shutil.rmtree(secret)
+
+        target, process, thread, bkpt = lldbutil.run_to_source_breakpoint(
+            self, 'Set breakpoint here', lldb.SBFileSpec('main.swift'))
+        log = self.getBuildArtifact("types.log")
+        self.expect('log enable lldb types -f "%s"' % log)
+
+        self.expect("expression s", substrs=['i = 23'])
+        self.filecheck('platform shell cat "%s"' % log, __file__)
+        # CHECK: LogConfiguration
+        # CHECK-NOT: secret
+        # CHECK: Import 

lldb/test/API/lang/swift/explicit_modules/bridgingheader/bridging.h

@@ -0,0 +1 @@
+#include "secret.h"

lldb/test/API/lang/swift/explicit_modules/bridgingheader/main.swift

@@ -0,0 +1,6 @@
+func main() {
+  let s = S(i: 23)
+  print(s) // Set breakpoint here
+}
+
+main()