[Tooling] Clear the PreambleSrcLocCache when preamble is discarded during reparsing

This ensures that diagnostics are not remapped to incorrect preamble locations after
the second reparse with a remapped header file occurs.

rdar://37502480

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@327322 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit 07fe671aaad3e93b9f7178ae1309304a73a3b3bf)

apple-llvm-split-commit: 1da4299dcc278446f356876e843423b55e4d9936
apple-llvm-split-dir: clang/

Author: hyp

clang/lib/Frontend/ASTUnit.cpp

@@ -1250,6 +1250,7 @@ ASTUnit::getMainBufferWithPrecompiledPreamble(
       Preamble.reset();
       PreambleDiagnostics.clear();
       TopLevelDeclsInPreamble.clear();
+      PreambleSrcLocCache.clear();
       PreambleRebuildCounter = 1;
     }
   }

clang/test/Index/Inputs/reparse-issue.h

@@ -0,0 +1,3 @@
+
+asdf;
+

clang/test/Index/Inputs/reparse-issue.h-0

@@ -0,0 +1,4 @@
+//
+//
+asdf;
+

clang/test/Index/Inputs/reparse-issue.h-1

@@ -0,0 +1,5 @@
+//
+//
+//
+asdf;
+

clang/test/Index/reparsed-live-issue.cpp

@@ -0,0 +1,4 @@
+// RUN: CINDEXTEST_EDITING=1 LIBCLANG_DISABLE_CRASH_RECOVERY=1 c-index-test -test-load-source-reparse 2 none -remap-file-0=%S/Inputs/reparse-issue.h,%S/Inputs/reparse-issue.h-0 -remap-file-1=%S/Inputs/reparse-issue.h,%S/Inputs/reparse-issue.h-1 -- %s 2>&1 | FileCheck %s
+#include "Inputs/reparse-issue.h"
+
+// CHECK: reparse-issue.h:4:1:{1:1-1:1}: error: C++ requires a type specifier for all declarations