[flang] Use precompiled parsing headers (llvm#130600)

Most of the high memory usage and compilation time in the frontend units
is due to including large parsing headers.
This commit moves out several of the largest parsing headers into a new
precompiled header linked to flangFrontend.
The new compilation metrics for FrontendActions.cpp are as follows:

User time (seconds): 38.40
System time (seconds): 2.00
Maximum resident set size (kbytes): 2710964 (2.58 GB) (vs 3.78 GB)

ParserActions.cpp:

User time (seconds): 69.37
System time (seconds): 1.81
Maximum resident set size (kbytes): 2599456 (2.47 GB) (vs 4 GB)

Alongside the new precompiled header compilation unit

User time (seconds): 41.61
System time (seconds): 2.72
Maximum resident set size (kbytes): 3107644 (2.96 GB)

---------

Signed-off-by: Kajetan Puchalski <kajetan.puchalski@arm.com>

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"]]
+)