Clang: Add more info about libclang to Readme.md

It was not mentioned that only mingw has profile-guided
optimization applied in prebuilt packages.
Also add the suggestion to use mingw version of libclang
in case MSVC compiler is used for build.

Change-Id: Icd824b83101dbebb6474e88c0f7bad3c6c52c61c
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@qt.io>

README.md

@@ -130,6 +130,9 @@ For detailed information on the supported compilers, see
          Code Model".
        * Set the environment variable LLVM_INSTALL_DIR to the LLVM/Clang
          installation directory if llvm-config is not in PATH.
+       * Before you launch Qt Creator you may prepend the PATH with
+         the location of libclang.dll/.so that you want to be used.
+         See more info in the section "Prebuilt LLVM/Clang packages".
        * When you launch Qt Creator, activate the Clang Code Model plugin as
          described in doc/src/editors/creator-clang-codemodel.qdoc.
 
@@ -212,10 +215,16 @@ Prebuilt packages of LLVM/Clang can be downloaded from
     https://download.qt.io/development_releases/prebuilt/libclang/
 
 This should be your preferred option because you will use the version that is
-shipped together with Qt Creator. In addition, the packages for Windows are
+shipped together with Qt Creator. In addition, MinGW packages for Windows are
 faster due to profile-guided optimization. If the prebuilt packages do not
 match your configuration, you need to build LLVM/Clang manually.
 
+If you use the MSVC compiler to build Qt Creator the suggested way is:
+    1. Download both MSVC and MinGW packages of libclang.
+    2. Use the MSVC version of libclang during the Qt Creator build.
+    3. Prepend PATH variable used for the run time with the location of MinGW version of libclang.dll.
+    4. Launch Qt Creator.
+
 If you use GCC 5 or higher on Linux, please do not use our LLVM package, but get
 the package for your distribution. Our LLVM package is compiled with GCC 4, so
 you get linking errors, because GCC 5 is using a C++ 11 conforming string