ClangFormat: Add caching of current style

Previously, ClangFormat was getting the style for a file
every time styleForFile was called, which could cause
unnecessary delays and impact performance. This commit
adds caching of the current style with a timeout of 1s,
so that ClangFormat can quickly access the cached style
instead of recalculating it each time.

Change-Id: I33c114d51d6ce1acd0b6d9d2a28e2b6712e149dd
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/clangformat/clangformatutils.cpp

@@ -31,7 +31,7 @@ using namespace Utils;
 
 namespace ClangFormat {
 
-clang::format::FormatStyle qtcStyle()
+clang::format::FormatStyle calculateQtcStyle()
 {
     clang::format::FormatStyle style = getLLVMStyle();
     style.Language = FormatStyle::LK_Cpp;
@@ -179,6 +179,12 @@ clang::format::FormatStyle qtcStyle()
     return style;
 }
 
+clang::format::FormatStyle qtcStyle()
+{
+    static clang::format::FormatStyle style = calculateQtcStyle();
+    return style;
+}
+
 QString projectUniqueId(ProjectExplorer::Project *project)
 {
     if (!project)