[flang] Use precompiled headers in Frontend

Reland of #130600 which was reverted on account of waiting for required
ccache compatibility fixes.

Signed-off-by: Kajetan Puchalski <[email protected]>

Author: mrkajetanp

flang/lib/Frontend/CMakeLists.txt

@@ -72,3 +72,11 @@ add_flang_library(flangFrontend
   clangBasic
   clangDriver
 )
+
+target_precompile_headers(flangFrontend PRIVATE
+  [["flang/Parser/parsing.h"]]
+  [["flang/Parser/parse-tree.h"]]
+  [["flang/Parser/dump-parse-tree.h"]]
+  [["flang/Lower/PFTBuilder.h"]]
+  [["flang/Lower/Bridge.h"]]
+)